Through all his years in Delhi politics Pranab Mukherjee likes to follow a rule - never returning to an office he has occupied in the past. So, when he was deputy chairman of the Planning Commission from 1991 to 1996, Manmohan Singh, then finance minister, used to drive down to Yojana Bhawan, the plan panel's headquarters on Parliament Street to attend meetings. That's because Mukherjee had occupied Singh's room in North Block, where the finance ministry is located, from 1982 to 1984 under Indira Gandhi.

On 25 October, 2006, however, Mukherjee was forced to make the first exception to this self-imposed rule - to return as the 14th external affairs minister to the room in South Block that he had vacated in May 1996.

Now after four decades in Delhi politics, he will be transgressing his rule a second time by stepping into the finance minister's office to take temporary charge till Prime Minister Manmohan Singh recovers from his second heart bypass surgery.

Singh has held the finance portfolio since the end of November after incumbent P Chidambaram was shifted to the home ministry following the Mumbai terrorist attacks.

Mukherjee's North Block return is expected Tuesday or Wednesday. As a result, he'll be working from two offices on opposite sides of Raisina Hill, since he retains charge of external affairs ministry.

"I'll definitely go to North Block and work from there. I'll devote some time to North Block every week," Mukherjee told Business Standard.

Mukherjee is no stranger to working from two offices. In Prime Minister Narasimha Rao's coalition regime, he handled the ministry of industry and commerce as well as Planning Commission.

After spending the day in Udyog Bhawan, the office of commerce ministry, Mukherjee generally visited Yojana Bhawan in the evening to work till late at night.

Like the prime minister, Mukherjee works long hours. He prefers to work on important government files at night when everyone is asleep. This routine will not change - he expects to adjust the extra responsibilities in his 15- to 16-hour working day.

Old-timers in the Commission recall how a peon, the worse for drink one winter night, accosted Mukherjee in the dingy corridors of Yojana Bhawan and admonished him for his unconventional schedule. "We can't go home because of you. Why can't you come and leave office on time?" he asked.

Meanwhile, in readiness for his new responsibilities, Mukherjee has already cancelled all his foreign tours except the Bangladesh visit.

"I had a series of tours starting with Mauritius. But now I can't leave the country. I'll only go to Bangladesh in the second week of February," he said. He will also not convene any out-of-turn cabinet meeting and the next one will be held as usual next Thursday.

Because the prime minister and Sonia Gandhi took the hurried decision to hand over charge of finance to Mukherjee, officials in the Prime Minister's Office said some gray areas remain in the administration. This is largely because the prime minister is deemed to be on sick leave and in the judgment of the government, no alternative arrangements need be made till he returns.

"When we go on sick leave, we don't make arrangements to clear files. After he returns from hospital, we expect the PM to slowly start working after resting another four or five days, so there is no need to delegate his authority yet since he will return to work after this sick leave to clear any emergency files," an official said.

In the event this changes, the prime minister will pass an order authorising delegation of powers to some other individual. This will then go the President of India for approval and will become official only after that.

The result is meetings and orders of the Appointments Committee of the Cabinet will be suspended for the time being. "There is no compelling need to hold an ACC meeting right away" an official in Cabinet Secretariat said.

Manmohan Singh has several ministries under his charge including atomic energy, coal and environment and forests. Around 16 officers of the rank of secretary report to him. So far there has been no delegation of work to the ministers of state. These officials were reporting to the PM directly. So till there is some delegation, these officers will take no decisions.

PMO officials are also confused about whom to report urgent matters to - but it appears the prime minister's Principal Secretary T K A Nair is holding daily meetings with Mukherjee. Cabinet Secretary KM Chandrasekhar also met Mukherjee.

On Thursday night, after his appointment as the West Bengal Congress chief was announced, Mukherjee reacted sharply to congratulatory messages and even asked his Additional Private Secretary Pradyut Guha to tell callers from the West Bengal unit that he hadn't returned from Kabul.

Within 24 hours, he was given charge of his favourite ministry but he made light of it.

"Arre bhai, this is nothing but some routine responsibility of the government. Don't get so excited," he kept telling people who wished him on his "promotion".

Before he officially took over additional responsibility, Pranab Mukherjee had already issued an order on Friday: there should be regular medical bulletins on the PM's health. That way, there is no scope for what Mukherjee hates the most - speculation.