Almost 18,000 more people with cancer in England could die after the coronavirus pandemic led hospitals to suspend treatment and deterred patients from seeking NHS care, research has found.

Cancer experts claim that an extra 6,270 people in England who have been newly diagnosed with the disease could die from it over the next 12 months as a direct result of the disruption caused by coronavirus, and the additional toll taking into account all those living with cancer could be 17,915.

That is an increase of 20% on the 89,576 deaths among cancer patients recorded annually in England, according to the latest available statistics.

The UK hospital death toll from coronavirus stands at 21,678, and more than 4,300 people have died in care homes in the past fortnight, bringing the total to more than 26,000 Covid-19 deaths since the pandemic took hold in Britain.

The new analysis sheds light on some of the wider health impacts of the pandemic. Macmillan Cancer Support described the findings as extremely worrying and said cancer should not become “the forgotten C” during the coronavirus crisis.

NHS England has launched a campaign urging people with symptoms of any potentially serious illness to seek help in the normal way by visiting A&E, a GP or dialling 999.

Urgent referrals by GPs for cancer tests have fallen by 76% and appointments for chemotherapy by 60% since the pandemic arrived in February despite NHS England insisting cancer treatment “should continue unaffected”. The health service hopes to resume cancer operations this week but faces a large backlog.

The new research, from University College London (UCL) and Data-Can, a health data research hub for cancer diagnosis and treatment in the UK, is the first to quantify the potential impact of the NHS delaying many forms of cancer treatment, including diagnostic tests and operations, and people not wanting to risk going into hospital in case they become infected. It is based on data from the health records of more than 3.5 million patients in England.

While some of the projected additional deaths will be among people with cancer who contract Covid-19, others will occur because diagnosis was late or treatment such as chemotherapy was delayed, the researchers conclude.

“Our findings demonstrate the serious potential for unintended consequences of the response to the Covid-19 pandemic, which may negatively impact on patients with cancer and other underlying health conditions,” said Dr Alvina Lai, a lead author of the study and a lecturer in health data analytics at UCL’s institute of health informatics.

The NHS should recognise patients affected as being particularly vulnerable and manage their care to reduce the risk to their health, Lai added.

Prof Peter Johnson, NHS England’s top cancer doctor, said on Wednesday that people with possible signs of cancer who delay seeking care could face serious consequences. He said they should have symptoms such as a lump or new mole checked out as soon as possible.

Patients can have cancer surgery at one of the NHS’s “Covid-free cancer hubs” already operating in 19 areas of England, he added.

Almost eight in 10 of the extra fatalities will be people recently diagnosed with cancer who also have at least one other long-term illness, such as heart problems, high blood pressure, obesity or diabetes, the NHS-funded study concluded.

Prof Mark Lawler, of Queen’s University Belfast and Data-Can’s scientific lead, said: “The results are concerning. We believe countries need to rapidly understand how the emergency is affecting cancer outcomes, otherwise we risk adding cancer and other underlying health conditions to the escalating death toll of the Covid-19 pandemic.”

Prof Harry Hemingway, a senior author and the director of UCL’s institute of health informatics, said: “The overall impact of the Covid-19 emergency on deaths in cancer patients could be substantial. There are many factors operating here including rapid changes to diagnosis and treatment protocols, social distancing measures, changes in people’s behaviour in seeking medical attention and [the] economic impact of Covid-19, as well as deaths due to Covid-19 infection.”

One patient with bowel cancer told the Guardian that he had had a CT scan but his planned colonoscopy – a diagnostic procedure – at a hospital in south-west England had been postponed for an unspecified period because the hospital’s endoscopy department had closed until the Covid-19 crisis eases.

“I am totally gutted to think I’ve been chucked on the pile in favour of coronavirus victims. For all I know they are giving me a death sentence [by] not looking at my case,” he said.

Another person said their mother’s surgery to remove a 3cm bladder tumour had been delayed for up to eight weeks because her hospital’s urology department had been shut as part of its preparations for Covid-19 cases. “This is worse than any nightmare I might ever have imagined in my darkest hour,” the relative said.

Macmillan’s chief executive, Lynda Thomas, said: “This research shows the possible extent of the direct and collateral damage caused by coronavirus on the health of our nation and to the lives of people living with conditions like cancer. It’s extremely worrying that we are likely to see an increase in deaths of people newly diagnosed with cancer.

“We are calling on all governments to stop cancer becoming the forgotten C and urgently ensure cancer services are getting everything they need to recover and catch up as quickly as possible from the disruption of the pandemic. This means having the right staff, protective equipment and testing to deliver safe care.”

Genevieve Edwards, the chief executive of Bowel Cancer UK, warned that unless NHS cancer services had the resources they needed to tackle the backlog of care, “the effects on the diagnosis and treatment of cancer will be felt for years to come”.