Britain's proposed illegal migration law will lead to more people missing vital medical treatment for fear of detention, large bills and deportation, charities and health workers say.
The Illegal Migration Bill, currently making its way through parliament, would prevent tens of thousands of migrants arriving in the country via irregular means from ever claiming asylum.
That would so bar them from free secondary healthcare, including operations or maternity care.
"This is going to result in an enormous health crisis and, of course, a mental health crisis," said Tara Povey, Policy and Research Manager at Refugee Action, a charity that helps and advises refugees and asylum seekers.
A Home Office spokesperson said the bill did not change the free access for all to primary care - accident and emergency and general practitioners - and said safeguards were in place "to ensure vulnerable people have access to medical treatment".
Under Britain's National Health Service (NHS), primary care is free for everyone, regardless of their immigration status.
But people without leave to remain in England are usually expected to pay for secondary care, including operations and maternity care. In Scotland and Wales, they can access free secondary care the same as other residents.
People who have been granted asylum and those applying for it can access free secondary care.
There are up to 1.2 million unauthorised immigrants in Britain, according to the U.S.-based think-tank, the Pew Research Center. They include those refused asylum by the Home Office and people who have overstayed their visas.
Many of these people avoid seeking medical help, exacerbating existing medical problems and leaving new ones untreated, refugee charities said.
"There are people who haven't seen a doctor for years ... because of the fear of being reported to the Home Office," Rita D'Alessio, who runs a free London clinic and helpline for refugees for the charity Doctors of the World (DOTW), told the Thomson Reuters Foundation.
That number, the charities fear, will rise if the Illegal Migration Bill becomes law later this year as expected.
Prime minister Rishi Sunak has said stopping asylum seekers arriving on the south coast of England - often in unseaworthy inflatable boats - is one of his top priorities.
But small boat arrivals soared to a record of 45,000 in the year ending March 2023, and that figure is expected to jump to up to 56,000 this year, the government says.
According to the Refugee Council charity, in the first three years after the Illegal Migration Bill becomes law, up to 257,000 people will have their asylum claims deemed inadmissible.
With no legal status, no right to work and no access to benefits or secondary healthcare, many would be driven to the margins of society, said Josephine Whitaker-Yilmaz, policy manager for Praxis, a charity that supports refugees.
"They will never be able to access any immigration status. It will dramatically increase the number of people that drop out of the system because they feel like there's no hope for them within it," she said.
"It's going to result in a massive increase in the undocumented, rootless, destitute and homeless population, which is already quite big."
DOTW said demand for its free London clinic and helpline had soared this year, especially from migrants housed in hotels seeking mental health support.
Clinic lead D'Alessio said the new law would leave irregular arrivals unable to claim asylum, without legal status and so more likely to bypass official channels for medical help.
"This new bill is stripping people's right to safety and there isn't really a plan to safeguard their wellbeing," she said.
DOTW said it was bracing for increased demand for its services and needed to more than double the number of its volunteer clinicians to around 150.
DOTW says it is a safety net for people outside the healthcare system, but it can only offer basic treatment and is not a replacement for the NHS.
The team - mostly volunteer medics - helps migrants register with their local GP and signposts them to other charities if they need financial or legal help.
D'Alessio said while some undocumented migrants could avoid the health system, pregnant migrants needed the NHS and that could put them in a precarious situation.
She said many pregnant migrants would delay ultrasound scans due to deportation fears and then would face fees starting between 5,000 to 7,000 pounds for vaginal deliveries, and more for complicated births, that they could ill afford.
According to a 2022 DOTW report, nearly half of all pregnant migrant women surveyed did not have critical antenatal care until after 16 weeks of pregnancy, compared to a national average of 10%.
If a migrant needed hospital care, the NHS says it might request information about their immigration status from the Home Office to see if the treatment was chargeable or not.
The Home Office said the NHS must provide treatment for any immediately necessary or maternity treatment where charges applied, even if payment had not been made.
A government spokesperson said data was shared between the Home Office and the NHS to determine if a treatment was chargeable and was not used for immigration enforcement.
The Home Office says the NHS must share information about a patient if they have an unpaid bill of more than 500 pounds ($622.20) and that could affect the person's immigration application.
Medact, a collective of health workers campaigning for equal healthcare access, said its members were concerned they were becoming proxy immigration officers.
"The NHS is increasingly being made into a wing of the Home Office," said James Skinner, a former nurse who now runs the group's 'Patients Not Passports' campaign.
"These can be life or death decisions that are being made about people on the basis of where they're from, not on the basis of what treatment they need," he said.