The prime minister has rejected calls to scrap the health surcharge for overseas NHS and social care workers.
Workers coming to the UK from outside the European Economic Area have to pay a fee to use the health service.
All adult migrants from outside of the EEA have to pay £400 a year – irrespective of whether they use the health service or not.
The immigration health surcharge (IHS) is going to be increased to £624 a year in October, with parents also expected to pay £470 a year for each dependent child.
The policy has been labelled "unfair and unjust" by opponents, with renewed focus being placed on it during the coronavirus outbreak given the numbers of overseas staff who work in the NHS and social care.
Dame Donna Kinnair, chief executive and general secretary of the Royal College of Nursing, said the union has received "devastating accounts from members who are struggling to pay the charge, and the impact that it is having on their families' lives".
She said there are a total of 77,065 non-EEA nurses who work in health and care nationwide.
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She added: "The current pandemic has served to reaffirm the importance of our internationally educated staff.
"Without them here, patient care would be at risk.
"This charge undermines the dedicated care overseas health and care staff provide to us all."
Labour leader Sir Keir Starmer raised the issue at Prime Minister's Questions, saying: "Every Thursday we go out and clap for our carers. Many of them are risking their lives for the sake of all of us.
"Does the prime minister think it's right that care workers coming from abroad and working on our frontline should have to pay a surcharge of hundreds – sometimes thousands of pounds – to use the NHS themselves?"
Boris Johnson replied: "I've thought a great deal about this and 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."
He added: "On the other hand we must look at the realities – this is a great national service, it's a national institution, it 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.
"So with great respect to the point [Sir Keir Starmer] makes, I do think that is the right way forward."