She drew rapturous applause this week when, introducing her husband at a make-or-break speech to his Labour Party, she embraced him in what newspapers dubbed a media-friendly 'kiss of life' for notoriously buttoned-down Mr Brown.

After that surprise appearance the couple travelled on to New York for the UN General Assembly - where she was pictured with US Republican vice-presidential hopeful Mrs Sarah Palin at a charity dinner which she co-hosted.

This higher profile suggests the former public relations executive, whose low-key style contrasts sharply with ex premier Tony Blair's forthright wife Cherie, is stepping out of the shadows just when her husband needs her most.

'She is charming and disarming,' said Australian novelist Kathy Lette, who has known her for 20 years, in the run-up to this week's Labour Party conference - to be followed by the Conservatives from Sunday.

'She is more disarming than a United Nations peacekeeping force and the Labour Party should unleash her.'

Mr Brown's Labour Party has been languishing around 20 percentage points behind the main opposition Conservatives, according to recent polls, and the party has been jumping with talk of rebellion.

Transport Secretary Ruth Kelly said she was quitting for family reasons on Wednesday amid reports, denied by Ms Kelly and Mr Brown, that the move is linked to dissatisfaction with the premier.

Four rebel lawmakers have already left junior government jobs this month and 12 have called for a leadership contest.

Commentators say Mrs Sarah Brown, 44, could help stop the rot by highlighting a more human side to Brown, whose dry, intellectual style turns many voters off.

'Sarah Brown is a phenomenal asset to Gordon,' Ms Zoe Williams wrote in the centre-left Guardian newspaper on Thursday.

'She makes him seem human; she makes his smile seem real; she makes you feel there is more to him than meets the eye; she makes you trust him, because she does.'

Her few public comments so far suggest she also has a dry sense of humour.

After press criticism of the outfit she wore when she met Carla Bruni-Sarkozy, wife of French President Nicolas Sarkozy, at Downing Street in March, she quipped: 'I didn't stand a chance, did I? I was standing next to a supermodel. Whatever I wore didn't matter.'

Mr Brown and Mrs Brown were married in 2000 in a modest ceremony at the then finance minister's home north of Edinburgh after he had spent years in the public eye as a bachelor obsessed with politics.

Their first child, Jennifer, was born seven weeks premature in 2001 and died 10 days later after suffering a brain haemorrhage.

The second, John, arrived in 2003, prompting Brown to say in a rare show of public emotion that he had 'the happiest family in Britain'. He added: 'I'm a father and that's what matters most. Nothing matters more than that. Nothing.'

Mrs Sarah Brown gave birth to another son, Fraser, in 2006, who was later diagnosed with cystic fibrosis, a life-threatening inherited disease which disrupts how the digestive and respiratory systems work.

She quit the public relations firm she co-founded soon after her marriage and now focuses on working with women and childrens' charities and her family.

But that could be changing, if her recent public appearances are anything to go by.

'Sarah Brown's passionate endorsement of her husband on the conference podium on Tuesday makes it impossible for her ever again to pretend that she is the quietly loyal political wife,' Ms Elizabeth Grice wrote in Thursday's Daily Telegraph.

The effect of her appearance 'was not just to galvanise Labour Party delegates but to make her essential to his survival; maybe even to Labour's.'

Mr Brown's conference speech, helped by his wife's introduction, was generally well-received.

A new poll on Thursday in the Sun newspaper put Labour only 10 points behind the Tories - its best showing in months, although it remains to be seen whether the poll bounce can be sustained. -- AFP