Hundreds of patients have been denied treatment for serious health problems including cancer, arrhythmia and cardiac chest pains after ministers forced the NHS to impose upfront charges on migrants deemed ineligible for free healthcare, the Guardian can reveal.

In one case, a patient with advanced stage cancer died after she went a year without treatment because an NHS hospital demanded £30,000 upfront to provide chemotherapy.

The first figures documenting the impact of the controversial new regime show that compelling patients who are often of limited means to pay in advance is leading to some going untreated.

Data sourced by the Guardian under the Freedom of Information Act found that across 84 of England’s 148 acute hospital trusts, 2,279 patients were charged upfront between October 2017 and June 2018. Of these, 341 patients across 61 trusts did not proceed with their intended treatments or appointments after being told to pay. The true figure across all trusts is certain to be higher, given that 64 trusts did not supply figures.

Many patients returned to their countries of origin rather than pay the charges. But of the 15 trusts that provided details of patients’ reasons for not proceeding with treatment, five trusts said that patients were unable to afford the cost.

These include Princess Alexandra hospital NHS trust in Essex, where a patient with arrhythmia disorders did not have the means to pay the charges, and Portsmouth hospitals NHS trust, where three patients, who needed radiology, andrology and breast treatment respectively, were unable to pay. Both trusts told the Guardian they had applied upfront charging correctly.

Dr Chaand Nagpaul, chair of the British Medical Association Council, warned that upfront charging risked worsening the health of already sick patients. “The current charging system lacks clarity around how and when overseas visitors should be charged,” he said. “It is vital that acutely ill people aren’t deterred from receiving care due to bureaucratic and financial obstacles, especially when this could result in patients becoming more ill, requiring emergency treatment and putting further strain on NHS resources.”

Since October 2017, NHS trusts in England have been required – as part of the government’s “hostile environment” approach to immigration – to seek advance payment before they provide elective care to overseas visitors and migrants who are ruled ineligible for free healthcare, such as failed asylum seekers and those who have overstayed their visa. Before last October, some NHS trusts charged upfront while others demanded payment after treatment had begun.

Treatment deemed urgent or “immediately necessary” is meant to still be carried out regardless of ability to pay. However, the Guardian has learned of a patient with an advanced form of blood cancer who was denied urgent chemotherapy after the hospital demanded upfront payment.

Elfreda Spencer, a 71-year-old Jamaican woman, was taken ill in 2016 while visiting her family in London. Tests at Hammersmith hospital diagnosed her with advanced stage multiple myeloma.

The hospital consultant advised that Spencer undertake chemotherapy as soon as possible. But Imperial College healthcare NHS trust, which runs Hammersmith hospital, demanded a £30,000 upfront deposit before it would start chemotherapy, and refused the family’s desperate offer to pay £500 a month to enable treatment to begin.

As a result, Spencer went a year without receiving any treatment, during which her cancer became terminal. She later received treatment at London’s Royal Free hospital, but died in January 2018 of sepsis. A spokesman for Imperial College healthcare NHS trust said it was investigating Spencer’s case.

In another case, a patient named Abdel, who had experienced months of chest pains, was discharged after being unable to pay £5,000 for surgery that clinicians deemed necessary to avert the risk of a stroke or heart attack. He went to another hospital, where the clinician recommended an urgent x-ray, but he was again discharged because he was ruled ineligible for free healthcare by the hospital’s overseas department.

Labour’s shadow health secretary, Jonathan Ashworth, called for upfront charging to be suspended. “Patients feeling they have no option but to deny themselves treatment because of Theresa May’s obnoxious ‘hostile environment’ agenda is not only leaving people who are ill without help but could potentially have wider public health implication as well,” he said.

“The Guardian earlier this year rightly exposed the dire and inhumane denial of treatment to a cancer sufferer caught up in the Windrush fiasco. These regulations have been implemented in a shambolic and cruel manner. They should be suspended while a full review is carried out.”

Medical charity Doctors of the World, which runs clinics providing free treatment to people excluded from mainstream healthcare, intervened on behalf of both Abdel and Spencer. Dr Katherine Taylor, the charity’s health adviser, said: “﻿The upfront charging regulations must be withdrawn and the government should, at the very least, carry out and make public the results of a human rights impact assessment of upfront charging.”

The government forced NHS trusts to impose upfront charges because of difficulties recouping costs from patients who received their treatment and then left the country. But critics view the requirement as part of the government’s “hostile environment” towards immigrants, although people applying for asylum or granted refugee status are exempt.

The Guardian’s FOI data shows that across 80 hospital trusts, the total amount charged upfront to patients – including most of those who did not proceed with treatment – was about £4m for the first eight months of the upfront charging regime. The government’s impact assessment from July 2017 predicted upfront charging would save the NHS £20m a year – less than 0.02% of NHS England’s £110bn total health expenditure in 2017/18.

Chai Patel, legal policy director for the Joint Council for the Welfare of Immigrants, said: “The government has no idea if upfront charging actually saves the NHS any money, because it is not measuring any of the costs of implementing the system.”

A Department for Health and Social Care spokesperson said: “The NHS is a national, not an international, health service. It is only right and fair that overseas visitors who need non-urgent care pay for it upfront if they want the treatment in the UK. Any costs recovered, including this £4m, is reinvested into frontline services so the NHS can continue to provide world class facilities.

“Our guidance is clear that urgent or immediately necessary care should never be withheld or delayed, regardless of whether a person can pay, and that decision can only be made by a clinician.”