Boris Johnson is facing a potential rebellion from some Conservative MPs after he rejected calls to scrap the fees overseas health workers have to pay to use the NHS.
The health immigration surcharge on non-EU migrants is £400 per year and set to rise to £624 in October.
BBC Newsnight understands that unease is brewing among Tory MPs over the prime minister’s stance.
Labour, the SNP and the Royal College of Nursing say the charge is “unfair”.
Labour plans to seek an amendment to the Immigration Bill to exempt NHS staff, including cleaners and care professionals, from the surcharge.
The Conservative chairman of the Commons public administration select committee, William Wragg, announced that he would support the fee exemption for NHS and care workers.
In a tweet, Mr Wragg said now was time for a “generosity of spirit towards those who have done so much good”.
Speaking at Prime Minister’s Questions on Wednesday, Mr Johnson said he had “thought a great deal” about this issue, given his own experience of being cared for by nurses from overseas when he was in intensive care with coronavirus.
Labour leader Sir Keir Starmer asked Mr Johnson whether he thought it was right that frontline healthcare workers were having to pay to use the NHS.
The prime minister replied: “I do accept and understand the difficulties faced by our amazing NHS staff and, like him, I’ve been a personal beneficiary of carers who have come from abroad and, frankly, saved my life.
“On the other hand we must look at the realities… [the NHS] needs funding and those contributions actually help us to raise about £900m, and it’s very difficult in the current circumstances to find alternative sources.”
Figures from the House of Commons Library, which compiles impartial briefings for MPs, showed £917m was the amount raised over four years by all migrants who have to pay.
The library estimated it would cost around £35m a year to exempt those who are NHS staff, although that figure would rise substantially for Labour’s plan to exempt care workers as well.
On Wednesday, the Royal College of Nursing called for the “grossly unfair” surcharge to be ended.
Dame Donna Kinnair, chief executive and general secretary of the union, said: “The government must drop this charge as a matter of urgency.”