AS THE undisputed heavyweight champion of the Labour party he famously stings like a bee; but floating like a butterfly has eluded him — until now. John Prescott is trying to become a nimble shadow of his former self, thanks to a gruelling exercise regime.

The deputy prime minister, who will be 68 next month, rises at dawn three times a week in an attempt to keep fit for the political battles of the post-Blair world. Loath to relinquish his job should Gordon Brown move into No 10, he is determined to ensure that his health can’t be used against him.

He arrives at the office gym by 7am and is soon puffing away on the treadmills or grappling with the rowing machines. He doesn’t bother with weights, preferring to stick to exercises that will improve his heart and lung fitness and — more importantly — shrink his waistline.

The effort seems to have been paying off. “Several people have commented that he is losing weight,” says an aide.

Mystery surrounds what he wears for his workouts. A series of corridors and atriums along Whitehall allows him to walk from the bedroom in his luxury state apartments at Admiralty Arch to the gym in the Cabinet Office without going outside.

Prescott’s official spokesman was coy when asked about his boss’s fitness regime: “It’s his business. I’m not saying if he wears a string vest, or cycling shorts, or whatever; it will only be turned into a joke. He doesn’t have a personal trainer; he just works out on his own.”

Professor Michael Lean, an obesity expert at Glasgow University, said: “He has to be careful and not do strenuous exercises as they could strain his back. It’s always commendable when someone tries to lose weight, but my best advice to Prescott would be to cut down on his eating.”

If his efforts pay off the Have I Got News For You? team might be forced to find somebody else to tease about their weight. Just last Friday a joke involving the deputy PM and a large meat pie prompted uproarious laughter from the BBC show’s studio audience.

In reality it’s probably a long time since he tucked into steak and kidney with abandon. Doctors warned him to change his diet when they discovered after the 2001 general election that he had type 2 diabetes, a condition linked to obesity.

Many MPs expect him to retire when Tony Blair leaves No 10. However, supporters say he refuses to contemplate stepping down. Under Labour’s election rules the contest for the leadership and deputy leadership do not have to be held at the same time, so there will be no automatic vacancy. Keen boxer that he is, Prescott is determined to fight his corner.

Harriet Harman, the solicitor-general, and Alan Johnson, the industry secretary, have also been mentioned as possible candidates.

But a source close to Prescott insisted: “He wants to serve under Gordon. Retiring is not on his mind.”

No minister seemed more adapted to the sedentary lifestyle than Prescott, who took a Jaguar ride from a hotel to the 1999 Labour party conference rather than walk a gruelling 250 yards. Yet as a young man he was active. He enjoyed swimming at school and later qualified as a diver.

As an opposition spokesman, he once swam underwater in a wetsuit along the Thames for two miles from Chelsea Bridge to No 10 to protest against the dumping of nuclear waste in the sea.

And he is famously keen on boxing. His famous left hook was unleashed to devastating effect in the 2001 general election campaign against a protester who was unwise enough to throw an egg at him.

Times recommends

• Live chat: Thrifty ways to update your winter wardrobe
• Is anybody out there really listening?
• Nutrition Q&A: Will eating late make me fat?