Overcrowding in hospital wards is raising the risk of superbug infections such as MRSA, it is claimed.
Busy wards may increase the infection risk
Liberal Democrat figures show 52% of trusts in England have bed occupancy rates higher than the recommended maximum safety level of 85%.
Among those, 95 trusts - more than one in five of the total - have occupancy rates in excess of 90%.
Experts say a rate above 85% raises the infection risk, but the Department of Health said the risk was low.
Figures from the Office for National Statistics show that deaths in England and Wales involving MRSA rose by 39% to 1,629 from 2004 to 2005.
In the same period deaths involving another bug, Clostridium difficile rose by 69% to 3,800.
A report drawn up last year by the Department of Health's chief economist found the most crowded hospitals - with bed occupancy rates over 90% - had MRSA rates 42% higher than average.
The Liberal Democrat figures suggest that occupancy rates in many geriatric beds are a particular concern.
Hospital superbugs are known to spread more quickly between elderly patients.
The latest figures show that of 261 NHS trusts with geriatric beds, almost 80% had occupancy rates of over 85%.
In 1997 the average bed occupancy rate was 80.8%.
The figures were compiled by the Liberal Democrats' health spokesman Norman Lamb.
He said: "If you are full there is more pressure, there is less slack in the system, it becomes much more difficult to keep patients clean, it becomes much more difficult to isolate patients.
"It makes it harder to fight the growing incidents of hospital-acquired infections which means that there are more people at risk of contracting infections and more risk of dying."
The Department of Health said that risk of contracting MRSA was "low".
A spokesman said: "The NHS treats a million people every 36 hours and there were approximately 7,000 MRSA bloodstream infections last year.
"The NHS is working hard to treat more patients and this means it needs to work even harder to reduce the risk of infection.
"Guidance was issued in the Chief Medical Officer's Winning Ways report, which required Chief Executives to ensure that infection control teams work with bed managers to optimise bed use while implementing procedures to minimise the risk of infection."
The figures were compiled from data for 2005-06 made available in September 2006.