Ricky Gervais has been criticised by disability groups for repeated use of the word "mong" on his Twitter feed.

The comedian's recent tweets have included phrases like "Good monging everyone", "Night night monglets" and "Two mongs don't make a right".

The word is sometimes used offensively about people with Down's Syndrome.

Mencap said using it could reinforce prejudice but Gervais insists the word has changed meaning and that he never meant to refer to people with Down's.

The Office star criticised "the humourless PC brigade" on his Twitter feed and said the term is now commonly used to refer to someone who is very stupid or idiotic.

We want people to know that such language can perpetuate discriminatory attitudes towards disabled people Mark Gale Mencap campaigns and policy officer

"The modern use of the word mong means 'dopey' or 'ignorant'," said Gervais in a statement. "It's even in modern slang and urban dictionaries."

On Sunday, he tweeted: "Well done everyone who pointed out that Mong USED to be a derogatory term for DS [Down's Syndrome], Gay USED to mean happy. Words change. Get over it."

However, disability charity Mencap called Gervais's tweets "very disappointing".

Campaigns and policy officer Mark Gale said: "When people in the public eye use words of this type [it] can be offensive to people with a disability and their families.

"We want people to know that such language can perpetuate discriminatory attitudes towards disabled people."

Down Syndrome Education International also said it was concerned and claimed many people would find it just as bad as offensive language related to race or sexual orientation.

It is also not the first time the 50-year-old has got into hot water for using the word.

During a 2010 stand-up show he used it to describe singer Susan Boyle, but following criticism again made it clear he was not intending to refer to people with Down's Syndrome.