Papworth Hospital, near Cambridge, launched an investigation earlier this month after a "small increase in early mortality" following operations carried out.

This year, seven of the 20 adult heart transplant patients have died within 30 days, the hospital said.

The Healthcare Commission carried out a review of procedures at the hospital and said its two-week review had found the quality of care was good and there were no common factors to explain the recent rise in the number of deaths.

Nigel Ellis, the head of investigations at the Healthcare Commission, said: "I want to be clear that we did not find any evidence of inadequate care or evidence that the deaths could have been prevented. But the number of deaths since January has clearly been high.

"This is why we are asking the trust to put in place a number of important checks and safeguards. With these in place, the public can be reassured that everything possible is being done to protect heart transplant patients.

"Heart transplantation is extremely high-risk surgery on very ill patients. In so many ways the Papworth Hospital transplantation service represents best practice and its good reputation is well deserved.

"Many of its patients would not be alive today without a transplant."

The Commission's team found that the time the transplanted heart was without blood supply during the operation was longer for those patients who died, but added that on average the times were lower than in previous years at the trust and were generally in line with the national average.

Stephen Bridge, Papworth's chief executive, said the hospital had now resumed its transplant service. He said: "We have a highly professional and dedicated transplant team and this report shows that we have many robust systems in place which contribute to the tremendous team effort that goes towards providing excellent care for our patients."