NEW DELHI - Prime Minister Manmohan Singh will Saturday undergo a bypass surgery that is expected to keep him out of work for up to four weeks, official sources said Friday. External Affairs Minister Pranab Mukherjee is likely to discharge his duties in his absence.

Manmohan Singh, discharged from the All India Institute of Medical Sciences (AIIMS) here Thursday following an angiography a day earlier, was re-admitted Friday after he felt uneasy at his 7 Race Course Road residence in the morning.

Extensive tests over the past two days revealed that the 76-year-old leader, who became prime minister in May 2004 at the head of a Congress-led United Progressive Alliance (UPA) government, had multiple arterial blockages.

Doctors attending to him said the coronary artery bypass graft surgery would be performed on him by surgeons also drawn from Mumbai’s Asian Heart Institute.

Following his angiography on Wednesday, doctors made it clear that the prime minister did not face any medical emergency. The prime minister had said he felt no discomfort while the tests were carried out.

‘The prime minister could have carried on for a few more days but his family felt the surgery should happen at the earliest,’ a senior official told IANS.

Medical experts said the prime minister would need to rest for two to four weeks after Saturday’s surgery.

Manmohan Singh underwent a bypass surgery in Britain in 1991. In 2004 he had an angioplasty to open clogged arteries. He had a prostate surgery in 2007 and underwent a cataract operation last year.

With Manmohan Singh getting hospitalised, the government decided Friday that his ceremonial role on the Republic Day Monday will be discharged by Vice President Hamid Ansari.

Ansari, officials said, would hand out the bravery awards and also place a wreath at the memorial to the unknown soldier.

Congress general secretary Digvijay Singh admitted that there was concern over the health of Manmohan Singh, whose six-year term in the Rajya Sabha expires in May 2013. ‘We are concerned about the prime minister’s health.’

Pranab Mukherjee, who has just returned from Afghanistan, will officiate in Manmohan Singh’s absence, official sources said.

Calling it ‘a normal routine thing’, he said he had discussed Manmohan Singh’s illness and ‘certain other matters’ with Congress president Sonia Gandhi.

But he quickly added: ‘It is premature to speculate. He is going for his treatment. We don’t know how long it will take, what will happen.’

Congress spokesman Manish Tiwari was equally circumspect, politely declining to say who will officiate the affairs of the country in Manmohan Singh’s absence.

‘Give India’s preeminent position, it is extremely unnecessary to second guess on an issue like this. Let us wait for an official announcement,’ he told reporters.

Pressed further, he said: ‘In the absence of the prime minister, there is a procedure that is laid down how affairs will be conducted. It has often happened in the past. Those very same procedures will come into play.’

Asked if Manmohan Singh would be the face of the Congress in the upcoming Lok Sabha elections, Tiwari said: ‘At this point of time, our top priority is to see that the prime minister recovers and recovers quickly.’

There were strong indications that Mukherjee, the seniormost member of the cabinet, would chair cabinet meetings in Manmohan Singh’s place.

Mukherjee is the leader of the house in the Lok Sabha. He also heads the Cabinet Committee on Political Affairs and the Cabinet Committee on Security. In the past, he has held fort when the prime minister has gone abroad.

But it is still unclear who will present the vote on account that sanctions pre-budget government expenditure. The prime minister might not be able to present the vote on account that is scheduled Feb 16.

This will need the parliament’s approval to finance government expenditure and pay the salaries of its employees till the next regular budget is presented after the general elections expected in April-May.

Usually, the finance minister presents it, but the finance portfolio is now with Manmohan Singh himself, after shifting P. Chidambaram to the home ministry.