Progress in clearing the NHS cancer treatment backlog in England has gone into reverse amid high Covid cases and staff shortages, analysis suggests.

With rising coronavirus hospitalisations also now piling pressure on the health service, experts have warned patients should brace themselves for worse to come as a “perfect storm” looms in cancer care.

The NHS has been striving to catch up with the pandemic backlog of cancer care but the analysis by Macmillan Cancer Support of official data suggests the drive has recently suffered a setback, with growing numbers of potential cancer diagnoses missed.

Four key cancer measures have fallen back, with two dropping to their worst ever recorded level.

Figures published by NHS England, and analysed by Macmillan for the Guardian, show the number of patients starting treatment in August following a decision to treat fell to 25,800. The figure was above 27,000 in June and July. The proportion of patients who began treatment within one month of the decision to treat fell to 93.7% – the lowest percentage ever recorded.

Data published last week also shows that in August there was a record-high number of patients forced to wait for more than two months after an urgent referral from their GP before they started cancer treatment. According to Macmillan, 4,075 patients only began treatment two months after being referred, the highest figure recorded.

A fourth measure – urgent cancer referrals by GPs – also slipped in the most recent month for which figures are available. The data shows 210,931 urgent cancer referrals were made by GPs in England in August. This is higher than the equivalent figure for August 2019, a non-pandemic year, which was 200,317, but Macmillan said it was a setback because it was lower than in June and July.

Macmillan said it was concerned that rising Covid hospitalisations were making it “even harder for the system to cope”. Efforts to tackle the backlog are also being hit by a shortage of cancer nurses, the charity said.

Steven McIntosh, the executive director of advocacy at Macmillan, said: “We know that many patients are entering an overstretched system that was on its knees even before the pandemic. This risks a perfect storm, as the system experiences a considerable influx of patients alongside an overwhelmed workforce struggling to provide the care and support that people urgently need.”

Minesh Patel, the head of policy at Macmillan, said: “It is extremely concerning to see signs that progress with clearing the cancer treatment backlog may have already started to falter. If the fragile cancer recovery now starts going backwards, the backlog will only continue to grow and people’s chances of survival will once again potentially worsen.”

He added: “It is disappointing to see a setback in referrals and lack of further progress in treatment numbers compared to last month.”

A shortage of nurses is compounding the crisis. Macmillan estimates that, even before the chaos caused by Covid, the NHS was already short of 2,500 specialist cancer nurses in England. That figure is now probably closer to 3,000, it said.

One in four people (25%) diagnosed with cancer in the UK in the past two years lacked support from a specialist cancer nurse, Macmillan said, with almost half of this group (44%) experiencing at least one potentially serious medical implication as a result.

The charity is calling on ministers to create a ringfenced cancer nurse training fund of GBP124m to train the extra 3,371 specialist cancer nurses it says will be needed in England by 2030.

McIntosh added: “With worrying months to come in the NHS, the government must act to both protect cancer services now, and address the shocking shortfall in cancer nurses in the upcoming spending review.”

A report by the Institute for Public Policy Research (IPPR) thinktank and the CF health consultancy published this month suggested it could take more than a decade to clear the cancer treatment backlog in England. The pandemic led to a 37% drop in endoscopies, a 25% drop in MRI scans and a 10% drop in CT scans than expected, it said.

The research also showed that during the height of the pandemic – March 2020 to February 2021 – 369,000 fewer people than expected were referred to a specialist with suspected cancer. There were also 187,000 fewer chemotherapy treatments and 15,000 fewer radiotherapy treatments.

An NHS spokesperson said: “The NHS has experienced six months of record cancer referrals, with the number of patients starting treatment higher than in August 2019 or 2020.

“The NHS remains open and ready to care for you so it’s important that people experiencing cancer symptoms come forward and get checked.”