Dr Sarah Wollaston, the chair of the Commons health select committee and an outspoken MP, said on Wednesday evening she had become convinced leaving the EU would damage the NHS.
She attacked Vote Leave’s claim Brexit would mean £350m could be spent on the health service every week.
"For someone like me who has long campaigned for open and honest data in public life I could not have set foot on a battle bus that has at the heart of its campaign a figure that I know to be untrue," Wollaston told the BBC.
But many claimed she had been bribed or was a "Remain plant" all along.
Pro-Brexit Tory MP Stewart Jackson even got involved, retweeting a claim that she was planted.
When challenged by Sky News' Faisal Islam over the retweet, he dismissed Wollaston’s move as “not significant at all”.
"She’s been making same claims for three weeks. Flaky but entitled to 5 mins of fame," he said on Twitter.
Others on were quick to join in...
Although not everyone was buying it, including Mark Wallace, the executive editor of ConservativeHome:
Wollaston also hinted on Thursday that other MPs were also considering defecting from the Leave campaign.
She told ITV's deputy political editor Chris Ship: "I think there are many people who have expressed their concerns...politicians just like the public are torn with this decision."
Wollaston told the BBC she had become convinced the NHS would suffer a "Brexit penalty" if the UK voted to leave the EU on June 23.
"If you’re in a position where you can’t hand out a Vote Leave leaflet, you can’t be campaigning for that organisation," she said.
She also tweeted:
In an interview with BBC Radio 4’s Today programme, she accused the Leave campaign of “post=truth politics”.
According to the Guardian, she added: “I can’t step foot on a battlebus or distribute a leaflet with information that I know to be untrue.”
Vote Leave’s argument that leaving the EU would free up £350m for the NHS has been criticised by the Commons Treasury as “highly misleading”.
The committee pointed out a substantial proportion of that figure already came back to Britain through the EU’s budget and as part of the UK’s rebate.
The UK Statistics Authority also said the claim was “misleading and undermines trust in official statistics”.
Her decision was welcomed by the prime minister: