The number of patients readmitted as emergencies within 28 days of being discharged rose from 359,719 in 1998-1999 to 546,354 in 2007-08, the data showed.

Elderly patients made up a large proportion of those affected - with 159,134 over the age of 75 readmitted in 2007-8, compared to 94,283 in 1998-99.

The Tories said the figures suggest patients are routinely being released before they are well enough.

The party's health spokesman Andrew Lansley said a Conservative government would change NHS rules to stop hospitals being paid again for treating patients they recently discharged.

He said: "It's staggering that there has been such a huge increase in the number of patients having to be readmitted to hospital as emergencies almost as soon as they've been allowed home.

"It's also a deeply worrying sign that the quality of care in hospitals is being undermined. This raises real concerns that patients are routinely being discharged too soon. Hospitals should not have an incentive to discharge patients quickly and then get paid by the taxpayer a second when they have to be readmitted.

"I will ensure that through our payment for results approach, hospitals have to meet any costs arising from emergency readmissions themselves."

However, a Department of Health spokesman said: "Patients are only discharged from hospital if the clinicians involved consider it safe and in their best interests.

"Some patients might require readmission if their health deteriorates, but the numbers are small. Only about 5% of patients discharged from hospital are readmitted within seven days of their discharge. Rates of readmission can also be a sign of better care. More people with long term conditions are being offered the choice of being treated in the community, with readmission only if the condition worsens."