Ryan Babel has been fined $15,890 for using Twitter to criticize referee Howard Webb.

STORY HIGHLIGHTS

• NEW: Manchester United's Rafael charged with improper conduct
• Ryan Babel is the first footballer to be charged by FA over a Twitter posting
• Babel used a mock-up picture of referee Howard Webb and made derogatory comments
• The Dutch international could face a ban if found guilty of charges

(CNN) -- Liverpool striker Ryan Babel has been fined £10,000 ($15,890) by the English Football Association (FA) for posting doctored photos of referee Howard Webb on his Twitter blog.

Babel was found guilty of improper conduct Monday by an FA regulatory commission and warned about his future conduct.

He is the first Premier League footballer to be charged and fined for "inappropriate" tweeting.

The Dutch international used his Twitter account to criticize Webb, who took charge of Liverpool's FA Cup third round match against Manchester United.

All participants need to be aware it is their responsibility to ensure only appropriate comments are used
--FA statement on Babel

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World Cup final referee Webb awarded United a disputed penalty and then sent off Liverpool captain Steven Gerrard as they slipped to a 1-0 defeat in Kenny Dalglish's first match at the Anfield helm.

Babel later posted a mocked-up picture of Webb wearing a Manchester United shirt and also wrote: "And they call him one of the best referees? That's a joke."

He later apologized and said it was meant to be light-hearted, but the FA took a dim view.

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Commission chariman Roger Burden said: "Social network sites must be regarded as being in the public domain.

"All participants need to be aware, in the same way as if making a public statement in other forms of media, that any comments would be transmitted to a wider audience.

"It is their responsibility to ensure only appropriate comments are used."

Babel had run into trouble on Twitter before, being forced to apologize to former Liverpool manager Rafael Benitez for comments he published after being dropped.

Meanwhile, Manchester United full-back Rafael has been charged with improper conduct by the FA for his outburst at referee Mike Dean after being sent off in Sunday's goalless draw at Tottenham.

Rafael was sent off for a second yellow card for a foul on Benoit Assou-Ekotto in the second half.