A blood test which looks for multiple types of cancer has failed to achieve its key objective in a major NHS trial, the company has announced.

The Galleri blood test is being trialled on 142,000 NHS patients with the goal of finding cancers early and saving lives.

However, the company behind the test, Grail, said there were still positive signs in the data that some of the most aggressive cancers could be prevented.

But researchers said any benefits of the test "remain speculative" until it was proven to save lives and the NHS said it was looking at the results "carefully". The blood test searches for fragments of DNA – or genetic code – that have escaped from tumours and entered the bloodstream.

The vision is to detect up to 50 types of cancer before the person has any symptoms.

At the start of the clinical trial, NHS England described the test as "the beginning of a revolution".

The trial was designed to see if screening people's blood would reduce the number of cancers being diagnosed at later stages when they may be harder to treat.

Stage three cancers are when a tumour has started to spread locally, stage four is when they have spread to distant organs in the body too.

The full data from the three-year NHS trial has not been published, but Grail has issued an update to investors.

It said: "While there was a trend towards reduction in combined stage three and four [cancers], the trial did not meet the primary endpoint. " Grail, a US pharmaceutical company, halved in value after the announcement taking its share price back to levels seen at the end of the summer.

However, a further analysis looking at stage four cancers alone showed they fell by around a fifth in the study, suggesting the most deadly cancers were being detected at an earlier stage.

How much attention should be paid to this result, when it was not the main focus of the trial, has become a major debating point.

Prof Charles Swanton, who is running the trials within the NHS, told the BBC he was "genuinely very excited" about the reduction in stage four cancers.

"I've been working in oncology for 20 years and the key, in my mind, is to reduce stage four because these are the cancers we cannot cure," he said.

However, other experts say the focus has to be on the primary endpoint as that is what the trial was designed to test.

Prof Richard Houlston, from the Institute of Cancer Research, said: "Without mortality data and a transparent account of harms, including false positives, unnecessary procedures, and opportunity cost, claims of population benefit from multi-cancer early detection remain speculative," he said.

The full data is due to be presented at a meeting of the American Society of Clinical Oncology later this year

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