Tory MP Sarah Wollaston has quit Vote Leave claiming the Brexit group's assertion that leaving the EU would free up £350m for the NHS "simply isn't true".
The health select committee chairman told the BBC: "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.
"If you're in a position where you can't hand out a Vote Leave leaflet, you can't be campaigning for that organisation."
The Totnes MP went on to say that Britain would face a "Brexit penalty" on the NHS as leaving the EU will hamper Britain's economy.
"The consensus now is there would be a huge economic shock if we voted to leave," she said. "Undoubtedly, the thing that's most going to influence the financial health of the NHS is the background economy. So I think there would be a Brexit penalty."
Last month, the UK's Statistics Authority lambasted Brexit campaigners for plastering the £350m figure across campaign literature and their battle bus. It said that the figure was "potentially misleading".
Email signatures of officials at the organisation read: "We send £350m to the EU every week. After we Vote Leave we can spend this money on our priorities like the NHS."
The EU referendum vote will take place on 23 June