Hospital infection rates have been studied

All 13 were said to be in a stable condition.

Scotland's health minister, Malcolm Chisholm, said he has called for an urgent report from the trust.

MRSA (methicillin-resistant Staphylococcus aureus) is a new generation of bacterial infection that is particularly resistant to antibiotics.

The infection can cause fever and pneumonia in frail and elderly patients and is primarily transmitted by contact, with poor hospital hygiene blamed in many cases.

A trust spokesman said enquiries were being carried out into the source of the bug, which was first noticed when more patients than usual developed post-operative infections.

He added: "The infection control team is working closely with clinicians and the Public Health Department of NHS Lothian to manage the situation and to ensure the safety and wellbeing of patients.

"The situation is being constantly monitored and the trust's infection control policy continues to be implemented.

'Stringent standards

"Early indications suggest that there is no single cause for this level of infection."

The trust said it was keeping the health minister informed about the situation.

Mr Chisholm said: "I have already ordered an urgent report to establish that everything possible is being done to investigate and control the spread of MRSA at the Royal Infirmary and limit the disruption to those cardiac patients who are being transferred.

"We have asked a series of urgent questions about how this has arisen and what is being done to control it."

Malcolm Chisholm has called for a report

Mr Chisholm said the Scottish Executive has been treating the issue of Hospital Acquired Infections "seriously" and was introducing new measures including the appointment of more infection control nurses and a hygiene audit of hospitals.

Scottish National Party health spokeswoman Nicola Sturgeon said cuts in cleaning services in hospitals have led to thousands of cases of hospital infection.

She said: "We have seen huge cuts in the numbers of NHS cleaners in recent years as a direct result of Labour's policy of privatising cleaning services.

"Hygiene standards have suffered because of Labour's obsession with putting profits before people and privatising public services."

Scottish Tory health spokeswoman Mary Scanlon said: "Stringent hygiene standards are the only way to minimise outbreaks of these sometimes highly dangerous bugs.

"I renew again my party's call for super sisters or modern matrons to be put in charge of every ward to ensure that the highest standards of hygiene are maintained."

Earlier this week, the Scottish Centre for Infection and Environmental Health (SCIEH) looked at the rate of infection in 18 acute hospital trusts.

The worst rates were found in hospitals in Lothian, Fife and Tayside.