Labour leader Jeremy Corbyn has denied calling Theresa May a "stupid woman" during Prime Minister's Questions.
He was asked to make a statement to MPs after facing Tory calls to apologise for the alleged insult.
Mr Corbyn said he was "opposed to the use of sexist and misogynist language in any form" and insisted he had actually said "stupid people".
Commons Speaker John Bercow said he had not seen the alleged incident and all MPs had to be taken at their word.
But Conservative MPs said they did not believe Mr Corbyn's explanation and repeated calls for an apology from him.
Mr Corbyn told the Commons: "During prime minister's question time today, I referred to those who I believe were seeking to turn a debate about the national crisis facing our country into a pantomime as 'stupid people'."
"Mr Speaker I did not use the words 'stupid woman' about the prime minister or anyone else," he added.
Conservative MP Rachel Maclean responded to his statement by saying: "Read my lips - I don't believe him."
The Speaker said he had reviewed television footage of Mr Corbyn's alleged insult, which had not been picked up by the microphones, and it was "easy to see why the leader of the opposition's words might be construed as 'stupid woman'."
He had also consulted a "lip speaker", who are employed to interpret lip movements in court rooms and other settings when a lip reader can't get close enough, who had lent weight to that view but had not been conclusive.
"Nobody can be 100% certain, that includes professional lip readers. But I will naturally take, and would be expected to take, the word of any right honourable or honourable member.
"It's reasonable to expect the House to do the same."
He added that Mr Corbyn had been "seated at the time and not addressing the House" so his words had not been "on the record".
Mr Bercow was separately accused by Tory MP Vicky Ford of calling her a "stupid woman" on an earlier occasion, as MPs turned the spotlight on the Speaker's own conduct.
The Speaker told Ms Ford she had not raised this point with him before, adding: "I refute it 100%"
Commons leader Andrea Leadsom had earlier reminded Mr Bercow of his failure to apologise to her for calling her a "stupid woman" during an incident earlier this year.
Mr Bercow said he had already dealt with that matter.
Mrs Leadsom said viewers and MPs would "draw their own conclusions" after Mr Corbyn's statement.
But Tory MP Desmond Swayne said criticising MPs for "what they might say under their breath" was moving "into the realm of thought crime".
And Labour MP Laura Pidcock said using the Commons chamber in this way was "absolutely pathetic".
"Stupid woman... stupid people... frankly neither is very nice"— BBC Politics (@BBCPolitics) December 19, 2018
Minister for Women Victoria Atkins calls on Labour leader Jeremy Corbyn to return to the Commons to explain his comments during #PMQs
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How did the row start?
Mr Corbyn had clashed with the prime minister over her Brexit deal, calling her decision last week to delay a vote on it a "deeply cynical manoeuvre" from a "failing" prime minister.
Mrs May hit back at Mr Corbyn, saying he had not tabled his promised no-confidence motion, and had then tabled one that was ineffective.
"I know it's the... pantomime season," she told MPs, "is he going to put a confidence vote? Oh yes he is," she said, prompting backbench Tories MPs to chant "oh no he isn't".
Continuing the pantomime theme, she told the Labour leader "look behind you - they are not impressed and neither is the country".
Mr Corbyn could be seen saying something under his breath in response.
Asked about what he had allegedly said by Tory MP Paul Scully, Mrs May said "I think that everybody in this House," particularly on the 100th anniversary of women getting the vote, should be encouraging women to get involved in politics and "should use appropriate language".
Former minister Sir Patrick McLoughlin used a point of order to accuse Mr Corbyn of having "muttered" that Theresa May was a "stupid woman".
Cries of "shame" and "disgraceful" were heard from the Tory benches at this point.
Sir Patrick added: "Would it not be appropriate for him to come back into this chamber and apologise?"
The row continued after Prime Minister's Questions, with a string of female Conservative MPs joining in with calls for an apology.
Former minister Anna Soubry suggested Mr Bercow would be more inclined to take action if the words had been uttered by a Conservative frontbencher at a female Labour MP.
Conservative Anna Soubry accuses Speaker John Bercow of double standards, saying if a male Tory MP had called a female Labour MP a "stupid woman" he "would take action immediately"https://t.co/psOR0Z7HN7 #PMQs pic.twitter.com/rUpJYcGnME— BBC Politics (@BBCPolitics) December 19, 2018
Veteran Labour MP Margaret Beckett suggested the accusations were party political:
Labour's Margaret Beckett calls scene in Commons an "orchestrated riot", as MPs demand Speaker John Bercow takes action over apparent Jeremy Corbyn jibe during #PMQs— BBC Politics (@BBCPolitics) December 19, 2018
Updates: https://t.co/eeUBTy8ETb pic.twitter.com/BOrSCXNZ4X
Lib Dem MP Wera Hobhouse said: "Sexism in any work place is not acceptable and it is time Jeremy Corbyn left the 1980s behind.
"Young women from up and down the country will have watched this exchange today and it is no surprise so many of them are put off participating in politics. If the allegations are true, Jeremy Corbyn must apologise."