Under pressure from his Western allies and facing a constitutional crisis at home, President Karzai of Afghanistan is expected to call a snap election for April 21 in an effort to wrongfoot his opponents.

Mr Karzai intends to make the announcement on Friday, The Times understands, despite a near-universal conviction among Western diplomats that it will be impossible to hold the poll in such a time frame.

At the moment presidential elections are scheduled for August 20 after the country's Election Commission ruled that they would have to be delayed from March or April — the deadline set by the constitution — because of logistical and security problems.

The President's term of office ends on May 21 and there have been increasing demands for him to stand down on that date and pass control to a caretaker government until the vote. Government officials say that the constitution is ambiguous and that Mr Karzai can remain in office.

By law, the elections should take place 30 to 60 days before May 21, but most of the country's upland areas will be snowbound for several more months, making this impossible — something that was apparently not expected when the constitution was drafted in 2003.

Diplomatic sources told The Times that Mr Karzai would call the snap election anyway unless a deal is done today with the opposition allowing him to stay on until August.

Waheed Omer, a presidential spokesman, said that Mr Karzai had not taken a final decision. “There is a lot of consultation ongoing — with the Cabinet, with parliamentary leaders, with the leaders of [political] groups. It is very important that the elections are seen as legitimate,” he said. “There is obviously discussion on whether he stays on after May 21. The President wants to ensure that he is seen as legitimate after that date.”

Any incumbent would have a clear electoral advantage in a patronage-based system where local powerbrokers and tribal leaders can bring thousands of votes with them. “Karzai feels that if he is not in office when the elections happen he won't win,” said one diplomat based in Kabul.

“Right now he controls security, the police, finances. He knows that it is a massive advantage. I think it is a shrewd move for him to call April elections.”

The decision to call a snap election may, however, be a political gambit, with Mr Karzai then expecting the election commission to insist on an August poll and allowing the President to counter claims that he is trying to violate the constitution.

It is believed that Mr Karzai will seek another term, although he has not formally stated his intentions. He has been rumoured to be under pressure from elements in the Government and the West to take on a “Father of the Nation” role and step aside, but such a move is thought to be unlikely.

So far 4.3 million voters — including 1.65 million women — have registered out of a potential electorate of 16 million people.

A “surge” of Western military forces is expected during the election period, on top of the 17,000 American troops already announced by the Obama Administration.

Nato commanders are looking for European nations to provide 10,000 extra soldiers for the vote. No leading political figures have put themselves forward for the presidency. A private phone poll of 6,500 voters indicates that Mr Karzai is the most popular figure likely to run, with about 15 per cent support.

He is followed by Ali Ahmad Jalali, a former Interior Minister, with 11 per cent; Younis Qanooni, the leader of the opposition, with 10 per cent; Ramazan Bashar Dost, an anti-corruption candidate, on 9 per cent, and Gul Agha Sherzai, the Governor of Nangarhar province, also on 9 per cent.

A senior Western diplomat described Mr Karzai as “down but not out” in the forthcoming election. There are indications from within the opposition that an anti-Karzai joint ticket is unlikely, but it is expected widely that a second-round vote will be needed.

Opposition sources said that they intended to pool resources behind whoever faces Mr Karzai in a run-off.

“Polling by the Asia Foundation last October suggested that up to 65 per cent of voters would follow the decisions of their community rather than their own conscience. Community decisions are typically dictated by tribal leaders and local powerbrokers, rendering opinion polls weak indicators."

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