Shadow Chancellor George Osborne faced a growing clamour to apologise last night after appearing to suggest that Gordon Brown was "faintly autistic".
Health campaigners and political opponents reacted with fury after Mr Osborne, at a fringe event at the Tory conference, appeared to joke that his counterpart suffered from the condition.
The jibe came as Mr Osborne talked about his own childhood, describing how his brothers nicknamed him "Knowledge" because he remembered so many facts.
The journalist conducting the interview interjected that he might have been "faintly autistic". Mr Osborne replied: "We're not getting on to Gordon Brown yet."
He later stressed that he had not actually used the words "autism" or "autistic" during the filmed session on Sunday, and insisted he did not believe the Chancellor had the condition.
However, the implication drew a response from Mr Brown's camp, with sources saying: "There's no need for him to apologise to Gordon Brown. But he should apologise to the thousands of people affected by autism for trying to turn their condition into a term of derogatory personal abuse."
The National Autistic Society said any perceived use of autism as a term to mock someone could cause "deep distress".
Carol Evans, a director of the charity, said: "Any pejorative use of terms relating to autism can cause deep distress and hurt to people affected by the condition.
"We as a charity are keen to raise awareness in order that these terms are not used lightly by commentators.
"To use such terms as a criticism of someone's social skills only perpetuates the confusion that surrounds the condition."
Writer Nick Hornby, whose 13-year-old son Danny has autism, condemned Mr Osborne for resorting to playground-style taunts.
The author helped co-found TreeHouse, an educational charity for children with autism.
He said: "George Osborne doesn't seem to have noticed that most people over the age of eight no longer use serious and distressing disabilities as a way of taunting people.
"Osborne claims that when he was younger, he was nicknamed 'Knowledge' - I'm not sure anyone will be calling him 'Knowledge' again for quite a while."
Labour's Janet Dean, chairwoman of the all-party Parliamentary Group on Autism and Burton MP, said: "I am shocked at the apparent use of the term autism as a method of political abuse."
Tatton MP Mr Osborne made a series of other personal digs against the Chancellor at the Bournemouth event organised by think tank Policy Exchange.
He claimed Mr Brown's speeches were not as powerful as Tony Blair's because he "wanted to be loved" by his audience, and expressed intense relief that the Prime Minister was stepping down.
He also revealed that the Chancellor was the only man outside his immediate family ever to hang up the phone on him - when they had been discussing "pairing off" for a Parliamentary vote.