Claims that Sir Keir Starmer and Boris Johnson had an "angry altercation" over COVID-19 vaccines in private after Prime Minister's Questions have been played down by allies of both party leaders.
Some reports suggested the pair had a "heated discussion" or a "tiff", and one Tory MP was reported to have claimed Sir Keir was "puce with anger" and "wagging his finger aggressively at the PM".
Minutes earlier, the two leaders clashed angrily at PMQs when Mr Johnson claimed Sir Keir had advocated that the UK should remain in the European Medicines Agency, prompting an angry denial from the Labour leader.
Sky News has established that the pair did continue their conversation about the UK's vaccine programme outside the chamber, near the entrance to the Aye lobby, after PMQs.
MPs have told Sky News that Sir Keir was seen talking to Mr Johnson in the lobbies and one senior Conservative MP confirmed he had walked past as the PM and Labour leader were speaking.
But the MP, a respected figure among members of both major parties, said: "It was not tense. Just two people having a chat."
And a Labour source later told Sky News: "They had a perfectly reasonable conversation as they often do after PMQs."
At a briefing for political correspondents, the prime minister's press secretary Allegra Stratton was unable to shed light on the conflicting accounts.
"I have seen the reports but I haven't seen the prime minister since he went to the Chamber so I can't confirm or deny that," she said.
According to early reports of the conversation that took place between Sir Keir and Mr Johnson outside the chamber, it became heated and Chris Matheson, a Labour MP and shadow minister, intervened.
Speculation about the nature of the leaders' conversation then grew when a Labour MP was reported as saying: "What happens in the lobby stays in the lobby."
But asked about claims that the exchange became heated and that Sir Keir was "rattled", Mr Matheson told Sky News: "It's absolute b*******. There was a brief chat and then Keir and I left together. Nothing more to it than that."
In the clash between Mr Johnson and Sir Keir over the vaccine programme during PMQs, the Labour leader looked visibly angry and snapped at the PM.
It began with the PM declaring: "If we had listened to (Sir Keir), we would still be at the starting blocks because he wanted to stay in the European Medicines Agency and said so four times from that despatch box."
A furious Sir Keir immediately responded: "Complete nonsense. Don't let the truth get in the way of a pre-prepared gag.
"The prime minister knows I've never said that, from this despatch box or anywhere else, the truth escapes him."
Despite Sir Keir's angry denial, Hansard, the official record of Parliament, shows that in January 2017 he questioned why the UK would want to leave the body and that it should be something retained as part of the Brexit process.
Later, in a significant climbdown, Sir Keir admitted he made a mistake in his response to Mr Johnson at PMQs over his previous comments on the European Medicines Agency.
A spokeswoman for Sir Keir said: "On a number of occasions the prime minister has wrongly claimed that Labour wanted to join the EU's vaccine programme. That is inaccurate and the claim has been found to be untrue.
"This afternoon during Prime Minister's Questions, Keir misheard the prime minister and assumed he was making the same false accusation again.
"Keir accepts that, on this occasion, the prime minister was referring to old comments about the European Medicines Agency and Keir admits he was wrong and made a mistake in his response.
"It's not Labour policy to join either the European Medicines Agency or the EU vaccine programme. We have never called for the UK to be in the EU vaccine programme. We remain committed to working with the Government to ensure we can be the first in the world to roll out the vaccine."