The true death toll from the hospital superbug MRSA may not be known, after figures from the Office of National Statistics showed that in 2003, almost 1,000 patients died from MRSA, methicillin-resistant staphylococcus aureus, and the number has doubled in four years.

The Government said the increase was partly because John Reid, the Secretary of State for Health, had requested stricter reporting of MRSA as the cause of death. But patients' groups, health unions and Michael Howard, the Tory leader, who has made MRSA a key election issue, said the real toll could be much higher.

The ONS survey showed the number of deaths attributed to MRSA increased by 155 to a total of 955 in 2003 compared to only 487 in 1999. Mr Howard and the unions said the figures could mask victims whose deaths were attributed to other causes such as pneumonia on their death certificates.

Mr Howard whose mother-in-law died of the disease, thought the true extent of MRSA in the health service could be higher than the official figure. "We know that people die and the cause of death is often vague and ambiguous," he said. "I think the figures may not be reflecting the true extent of MRSA." Mr Howard, quoting earlier figures by the National Audit Office, said 5,000 died each year from MRSA and other infections picked up in hospitals. He added: "No other country has seen the superbug infection take over its hospitals in the same way we have in Britain."

He accused Tony Blair of being personally responsible by imposing health targets that prevented matrons from closing infected wards. "Mr Blair's obsession with targets has created a culture in which the superbug thrives," he said. "It is a fact that doctors and nurses are prevented from closing wards they know to be infected with the superbug because of Mr Blair's targets."

Tony Field, chairman of the patient group MRSA Support, said the health authorities "still haven't got to grips with it". He added: "I would expect the true figure to be much more than 955." Karen Jennings of the NHS union, Unison, said: "It cannot be a coincidence that, just as the number of cases of MRSA have gone up, so the number of cleaners employed in the NHS has gone down."

The Liberal Democrat health spokesman Paul Burstow said the figures were "only the tip of the iceberg". Chief Nursing Officer Chris Beasley said "Improving reporting of MRSA will help us identify avoidable factors and learn useful lessons." Last night, Mr Howard's allegations were rejected by allies of Mr Reid, who said the Royal College of Nursing had said closing a ward and dispersing the patients could spread the disease. "Matrons do have the power they need," said a source close to Mr Reid. "This is a good sound bite for the Tories, but it is a lousy policy."

Mr Reid has announced measures, including a campaign to get patients, visitors and nurses to wash their hands, and the introduction of a target to halve blood-borne MRSA. But Mr Reid is irritated with the attacks by Mr Howard, who appears to have put his finger on a growing public concern.