Downing Street diet: The incredible shrinking Prime Minister

Healthy break leaves PM a stone lighter, but Tory leader seems to have put on weight

He may be fighting a losing battle with malcontents in his party, but Gordon Brown has had one success this summer – losing more than a stone.

The Prime Minister's weight loss was so noticeable that Nicolas Sarkozy spent some time at last week's summit in Brussels pointing it out to other European leaders.

Before he took his first proper holiday as premier in August, Mr Brown's Gieves & Hawkes suits appeared a little tight around the waistline. Yet the latest pictures show there is now more room to breathe in the bespoke tailoring.

During the four-week break in Suffolk and Scotland, Mr Brown took on a £50-an-hour personal trainer, Millie Dobie, who took him for pre-breakfast jogs – with police protection officers jogging behind.

The Prime Minister also took long walks with his wife, Sarah.

But another reason for his new svelte look has emerged, with an insider revealing that the break has kept Mr Brown away from the tempting high-calorie snacks that sit in bowls on the Downing Street tables and sideboards.

Long hours and a sedentary office lifestyle meant he would crave the instant sugar-rush of the sweets, the insider said. "Everywhere you turn there's a bowl of KitKats or biscuits waiting to be eaten," the source added. "It's very tempting to keep snacking when you're working long hours."

However, another reason could be the toll that worrying about the economy and Labour's dwindling support in the polls has taken on his body.

Mr Brown, 57, regularly uses a running machine in No 10, as his predecessor Tony Blair did before him. Mr Blair put on weight during the 2005 election campaign, when a bad back meant he could not exercise regularly, but for the rest of the time was "near-obsessive" about working out.

The former PM took his treadmill with him when he left, but the new one was installed last year at the request of Mrs Brown, who wanted to lose weight.

Last year, the Prime Minister's wife dropped from a size 16 to a size 12 after taking up a healthy eating and fitness regime, and she encouraged her husband to follow suit.

It was once revealed that Mr Brown listens to Leona Lewis on his iPod while running, an exercise the Prime Minister has otherwise described as "boring".

During Mr Brown's trip to the Beijing Olympics last month, his wife proudly showed off her husband's trim new look. It is believed that his suits, which are all from Gieves & Hawkes of Savile Row, have not been taken in, but simply fit better than they did before.

Downing Street refused to comment on the total weight loss and has denied that Mr Brown has had a makeover. But it is thought that he has lost more than a stone. However, Mr Brown has once before tried to give up KitKats, his favourite chocolate bar, without lasting success.

While the Prime Minister has been shedding the pounds, it has been noted that his Conservative opponent has been putting on a little weight. This was prompted by a picture of a T-shirtless David Cameron on board a yacht off the coast of Turkey last month.

An aide insisted that the Conservative leader had not piled on the pounds, and blamed an unfortunate camera angle on the apparent weight gain. And the aide added that Mr Cameron, who also regularly cycles to Westminster from his home in west London, was playing tennis twice a week in the run-up to next month's Conservative party con-ference in Birmingham.