Gareth Van den Bogaerde breaks his family's 40-year silence to reveal an intimate picture of his film star brother. Jan Moir reports

In a hotel in the New Forest, teacups rattle in the lounge as a soft, summer rain falls on the lawn outside. A mobile phone rings. Someone slips a few biscuits to a yapping dog. It is not the best place in the world to conduct an intimate conversation about one of Britain's most famous actors, but Gareth Van den Bogaerde and I do our best. "Do you think he was jealous of your heterosexuality?" I ask, as the crockery falls silent.

"Partly. I think it has to be partly that, yes," he says. "But, really, he resented my advent. After that, he resolved that he would build a shell that would never be pierced; he would never be hurt by anyone again. He remained in that armour for the rest of his life, you know."

The endless homosexual concealment, I say, must have been stressful.

"Oh, yes. At the time that Dirk was sexually active, the law was such that it could not come out. If it did, he could have gone to prison. It couldn't be admitted," he says, as even the dog goes quiet. No wonder, for Gareth Van den Bogaerde has quite a tale to tell.

Many years ago, his brother Derek simplified his name to Dirk Bogarde and became one of our biggest post-war film stars. Born to a father of Flemish background who became art editor of The Times and a mother who was a slightly successful Scottish actress, the brothers had a jagged, difficult relationship, although the family were always discreet about this and other matters. For example, they all knew that Dirk was gay, but managed to keep a lid on it for years. As Van den Bogaerde says, you had to, in those days.

In conversation and on the telephone, they always circumspectly referred to Bogarde as D or Uncle D and they never talked to journalists. Gareth once accidentally did, when he worked as a film editor at Pinewood in the 1960s, when Bogarde was a big star there, trundling through the studio gates in a Rolls-Royce every morning. His little brother took the bus.

"I got terribly caught, I had no idea. It was a terrible con. In all innocence, I thought it was terrific; I was being interviewed about my brother!" says Gareth. "But when it came out, Dirk was absolutely beside himself and saw it as a betrayal. He said to me: 'Don't ever speak to the press again.' And none of us ever did."

Until now. Although the Van den Bogaerde family are thrilled with Dirk Bogarde: The Authorised Biography written by former Daily Telegraph literary editor John Coldstream, and in print at the end of this month, they are upset that extracts published in a tabloid newspaper show a tainted, unbalanced picture of Bogarde which depicts him as a fantasist capable of "malice, vile cruelty" and every other sin under the sun.

"That is the tragedy. If you take the bits that are beastly and string them all together, of course you end up with a black picture of someone. We all have dark sides of our life. But Dirk had extremes of kindness and generosity. He was a tremendously sparkling character of quick mind and brain," Gareth says of his brother, who died of a heart attack in 1999.

Today, 70-year-old Gareth unzips his cosy fleece, looking every inch the hearty boatie that he is, although there are cobwebs of sadness around his eyes. He has enjoyed a successful career making commercials and documentaries and his voice is clear and gentle, deceptively so, because sometimes the soft cadences disguise the frankness of what he has to say. Being the sibling of a rich and famous person is never easy – they unbalance the family unit, they suck up all the oxygen – and for Gareth Van den Bogaerde, it was more difficult than most.

"We had a love/hate relationship," he says of his brother. "I loved, he hated. I don't say this flippantly, it is the truth. I did love him, but he hated firstly himself and, secondly, what he had done, which was to nurture resentment of me. I think he had hurt himself because of that and he hated it. He hated himself."

And Bogarde did hate, there's no question about that. The man who would go on to star in films such as Death in Venice, The Servant and Victim, the author who wrote seven volumes of memoir, a handful of novels, reams of journalism and was knighted in 1992 could never forgive his baby brother for... what exactly? For being born, seems to be the answer. Dirk – or Derek, as he was still known then – was 13 years old when Gareth arrived in the bosom of the family and later wrote how he was "silent with shock" when he first saw the baby. He sensed that "someone had turned my egg timer upside down and the sand had started to run the other way".

Shortly afterwards, his father drove Derek to Glasgow – away from his beloved mother, his toy theatre and his friends – to stay with relatives. The new baby had made conditions in the family's north London home cramped and, on top of this, Derek was doing badly at school. Forced to stay in Scotland for three years – something he could never forgive Gareth for – he returned to England as a 16-year-old at the end of 1937. According to his biographer, he had become "less carefree, more capable of petty cruelties".

Now it was baby's turn to suffer. When little Gareth leant out of a sash window, Derek came up behind him and brought it down on his fingers. His mother came running when she heard the scream, but he told her: "It's all right, I'm just squeezing a thorn out of his finger."

In later years, his tortures became more exquisite, like the time when Gareth met Elizabeth Taylor when he was about 14. "She was with Michael Wilding," he says. "She was the most beautiful thing I had ever seen, I couldn't take my eyes off her. I was very gauche. And then Dirk introduced me by saying: 'This is my small brother, he is a farm boy'."

On another occasion, film star Dirk had just discovered an exciting new aftershave called Old Spice and had bought four cases of it. Sleek and perfumed in his dinner jacket, he wafted down the stairs past poor Gareth on his way out to an engagement.

"God, that smells terrific," said the awkward but excited teen.

"Yes, it is," sneered Bogarde in front of everyone. "And Jesus, you could do with some of it."

He could be wicked.

"Oh, painfully hurtful," says Gareth. "Those sort of put-downs were so common and I would be deeply hurt by things like that, because at 15 or so you just crumpled up. But as I grew older, I thought, 'Ah, you are such a clever bugger'. And I began to understand what I must have represented to him, why I stood in the light in front of him. I had to be there, I couldn't avoid it. I understood why he felt the way he did. It was damned unreasonable, but I understood it."

The fact that Gareth Van den Bogaerde can find it in his heart not just to forgive his brother, but also go some way towards understanding and appreciating him, says much about him and the depths of his fraternal love, but also a great deal about Dirk Bogarde himself. He was a difficult, brittle, brilliant man who, even at his most repellent, had the ability to make others love and indulge him.

Coldstream reluctantly points out that many of the assertions in Bogarde's seven volumes of autobiography were – how can we put this? – completely made up. His claim, for example, that he was in the Belsen concentration camp shortly after it was liberated – complete with lavish descriptions of scabbed inmates and how it affected him for ever – is tactfully dismissed by Coldstream.

The prevailing winds seem to suggest that Bogarde dissimulated like this because it was all part of a great smokescreen and intricate camouflage that he felt compelled to throw up around himself. It is a generous explanation; others just might call what he wrote plain, boiled lies.

Yet it would take a hard heart not to sympathise in some respects. Despite Bogarde's homes in the south of France, his lovely cars, good food and wines, burgeoning fame and the deeply happy, 40-year relationship with his friend Tony Forwood, always standing right in front of him was Gareth, who, with his wife and five children, had everything that Dirk really wanted. But he had a funny way of showing it.

"He adored my children, although he refused to see them until they were 14. He hated them so much when they were small. He used to say: 'Don't bring those filthy, little shits to my house. I don't want anything to do with them'," says Gareth. "It was hurtful, until I began to appreciate what was eating him."

When Gareth's firstborn son, also called Gareth, died of pneumonia when he was 21, Bogarde tried to help, in his own quietly horrified way. He was upset, of course, but his first reaction was to urge everyone to forget all about it, and he sent over a large cheque for Gareth to take the whole family away on holiday to get over it.

"He was shocked, but his shock was that such an amazingly awful event, such an event of huge magnitude, happened to me and not to him. I always felt that he rather resented Gareth being my son only because it was such a big event, such a huge thing to happen."

Bogarde did not attend the funeral.

"He had an appointment with Vogue in Paris. I didn't expect him to come. He just said, 'I can't come. I've got this thing going on with Vogue, I can't get out of it.' But also, he knew that wherever he appeared, that was where the attention was."

The truth about Dirk, as his family might argue, is both lighter and darker than it appears at first glance and certainly, for Gareth Van den Bogaerde, the publication of this new biography has brought a kind of catharsis and a lot of relief.

"It has been hugely emotional," he says. "It lays out a life for me that I had only partially understood. I've got the remaining pieces of the jigsaw puzzle, which I have now put together. All that angst and those hurts can be put to rest. I understand Dirk now more than I ever thought I would."

• Dirk Bogarde: The Authorised Biography by John Coldstream (Weidenfeld & Nicolson, published on September 13) is available from Telegraph Books Direct for £18 plus £2.25 p&p. To order, call 0870 155 7222