I take fatherhood very seriously, says Brown as Cameron claims 'overworked' PM should spend more time with his family

The Prime Minister has not been on a family break since taking power a year ago from Tony Blair - famous for high-profile twice-yearly sunshine breaks at the foreign homes of famous friends.

Mr Cameron, who went to South Africa with wife Samantha and their three children in February, warned his rival he risked becoming a 'decision-making machine'.

'I think you have to do it to stay on top of the game. If you immerse yourself from 5am until 11pm, it so affects your balance, family life, your sense of who you are,' he said.

'You lose your judgment. If you are tired, you make bad judgments  -  simple as that.

'The decisions you make are inevitably partly based on who you are as a character, but you need reasonable balance and equilibrium to remind you of who you are.

'If who you are at the core is a husband and family man, you can't allow your work to morph you into something different: a decision making machine.'

He said he had been tempted to tell the Prime Minister, when facing him across the Commons chamber recently: 'For heaven's sake, go on holiday.'

'I thought about it again when I was walking through this unbelievable valley in Spain, and I thought: "you really do need some time when the head can empty, whoever you are."'

Asked if Mr Brown would take the advice and go away on holiday with his wife Sarah and two young children, his spokesman said: 'The Prime Minister takes his responsibilities as Prime Minister very seriously and is committed to working very hard on behalf of the people of this country.

'He also takes his responsibilities as a father very seriously and tries to manage the two - as do millions of parents in the country.'

Mr Cameron's remarks, in an exclusive interview with the Mail,  dramatically broke with political convention.

Breaking with convention: David Cameron believes Gordon Brown should spend more time with his family

His intervention will put the spotlight back on Mr Brown's personality.

It came as a new poll showed the Premier with the lowest approval level of any party leader since ratings were first taken in 2003.

The Populus survey in the Times also gave the Tories a 20 per cent lead.

Mr Cameron also revealed in the interview that  his marriage hit a 'danger point' after the birth of his first child Ivan, who has cerebral palsy and severe epilepsy.

He said he and his wife Samantha were plunged into 'mourning' when they realised how ill the baby was, and agonised about whether to have more children.

'We talked about it and we were very clear that we were going to find a way through, but it was very difficult. We crossed our fingers and prayed,' he said.

His openness-about his domestic life will be seen as a direct challenge to the notoriously private Mr Brown, who is under pressure from his supporters to soften his image.

• Alistair Darling said yesterday he realised as soon as he became Chancellor that Mr Brown's decision to scrap the 10p tax band would hit millions, but he could not persuade Mr Brown to take action.