Jim Wallace stepped into the breach

The operation could take about two to three hours, Mr Dewar's surgeon, consultant cardiologist Dr Kerry-Jane Hogg, said.

Mr Dewar, the MP for Glasgow Anniesland, will then need to spend about 10 days in hospital recovering from the surgery, which will be done on the NHS, before convalescing at home for between eight and 12 weeks.

The first minister's official spokesman, David Whitton, confirmed that Mr Wallace, leader of the Scottish Liberal Democrats - minority partners in the Scottish coalition administration - will take charge during Mr Dewar's absence.

Mr Dewar was discharged from Glasgow Royal Infirmary on Wednesday after an overnight stay for tests.

Prime minister notified

Looking slightly tired but otherwise healthy, Mr Dewar told reporters: "I'll be going back to work tomorrow."

It was expected that the first minister would spend Wednesday resting at home.

But it was anticipated that he would be back at the Scottish Parliament in time for Thursday's crunch debate on proposals to repeal Section 28 which bans the promotion of homosexuality in schools.

Prime Minister Tony Blair was notified in advance by Mr Dewar that he would be going into hospital.

David Whitton: Outlined contingency plans

Some of Mr Dewar's senior colleagues were told in advance, but others only learned at 0930BST on Tuesday when they assembled for the start of a Scottish Cabinet meeting in Edinburgh.

Deputy First Minister Jim Wallace took over Mr Dewar's duties in the meantime.

Since the launch of the new parliament last year Mr Dewar has been at the centre of several high-profile controversies including the cost of the new parliament building, the planned repeal of Section 28, and student tuition fees.

Between the cardiac irregularity being picked up in March and Tuesday's further tests, Mr Dewar's schedule had included a trip to Japan, and a key speech to the Scottish TUC in Edinburgh last week.

It is not known exactly how long Mr Dewar will take off work but it is expected he will be able to read ministerial papers during his convalescence period, which is likely to coincide with the parliament's summer recess.