David Halpern, the director of Number 10's Behavioural Insight Team, said not having someone with whom to share problems was one of the most significant lifestyle factors affecting mortality.

Dubbed the "nudge unit", Mr Halpern's team was set up to develop ways to push people gently into changing behaviour rather than more draconian government interventions.

Mr Halpern was picked by Prime Minister David Cameron as one of six experts joining him at a summit of Nordic and Baltic states, where one topic was how to ensure more workers delayed their retirement.

He told other leaders and experts that a majority of the UK's over-75s considered themselves lonely "all or most of the time".

"Work matters, particularly for older people, not just for money, but absolutely for social contact," he said.

Presenting a graphic setting out specific lifestyle factors, he said: "We know smoking is really bad for you. But much worse are things like social relationships.

"If you have got someone who loves you, someone you can talk to if you have got a problem, that is a more powerful predictor of whether you will be alive in 10 years' time, more than almost any other actor, certainly more than smoking."

He also suggested the numbers of old people living alone were causing the UK's housing shortage.

"We do have enough houses, it's just essentially they are lived in by older people."