Harriet Harman will fill in for Brown at Prime Minister's Questions next week

Mr Brown is out of the country attending the NATO summit in the Romanian capital of Bucharest.

So he has handed responsibility for the showpiece event of the Commons week - when the PM is traditionally held to account by MPs - to Ms Harman.

It will be the first time the Leader of the Commons has stepped into her bosses shoes to answer questions about Government policy in the 30 minute session, renowned for its unforgiving boisterousness as MPs taunt and jeer.

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However, the decision will raise concerns among Labour who are aware of Ms Harman's capacity to place herself in trouble.

In October, she made an unfortunate blunder by publicly accepting the possibility that Mr Brown's decision to back away from an early election had harmed his reputation.

And on day one of her new job in June she appeared to perform a u-turn on her views over the Iraq War - angering people who supported her because she suggested she would apologise for the Iraq War during a close-fought deputy leadership campaign.

She was also embroiled in the 'dodgy donations' affair when it was revealed she accepted a £5,000 gift from a woman who was a go-between for controversial businessman David Abrahams. She insisted she had not realised and gave back the money.

Because Mr Brown is not there, Tory leader David Cameron will also step aside. Instead Ms Harman will face Tory foreign affairs William Hague or Shadow Commons Leader Theresa May - which would make it the first time two women had faced each other across the Despatch Box.

Mr Brown's decision is likely to have put Justice Secretary Jack Straw's nose out of joint.

He is the senior member of Cabinet who is privately said to regard himself as the de facto Deputy Prime Minister.

However, sources close to the Prime Minister made it clear Mr Brown would not be handing over Britain's reins to Ms Harman while he attended the three-day summit.

One insider said: 'She is not the Deputy Prime Minister and she is not running the country for Gordon Brown. He remains Prime Minister and he remains in charge.'

Before there was the post of Deputy PM, it was normal for the Leader of the House to stand in for the Prime Minister if they were away on business.

Both Robin Cook and Geoff Hoon stood in at PMQ's during Tony Blair's decade in Number 10.

But Mr Blair had no qualms putting blustering John Prescott in charge of the country while he was on holiday.

On one occasion in 2006, Mr Blair left discredited Mr Prescott - reeling from revelations of his affair with his diary secretary Tracey Temple - at the helm while he had a three-week sunshine holiday at pop star Sir Cliff Richard's luxury villa in Barbados.

It caused a chorus of disapproval because it left Mr Prescott in charge of delicate negotiations to tackle a crisis in the Middle East.