The chair of the Health Select Committee said she could not support the claim that the UK could get back £350m per week from its EU budget contributions and spend the money on the health service.
"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," the former GP told the BBC.
"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 argued that, far from a financial bonus from taking money back from Brussels, leaving the EU would hit NHS funding.
"The consensus now is there would be a huge economic shock if we voted to leave. 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."
Her announcement was hailed as a "powerful intervention" by David Cameron.