UPDATE: David Cameron accused the Liberal Democrats of making unaffordable promises to voters while in Brighton today - comparing their manifesto sums to a sudoku puzzle.

Answering questions from sixth-form students at Varndean College in Brighton, he focused his criticism on proposals to reduce income tax for most workers by closing loopholes exploited by high earners.

"It's all very well to say 'I've got numbers at the back of my manifesto'," he told the packed hall.

"I mean, there's numbers in a sudoku but it doesn't mean it all adds up.

"They are saying the first £10,000 of your income, you shouldn't pay any tax on at all. That is a beautiful idea, it is a great idea.

"How do you pay for it? It is £17 billion.

"We have this enormous deficit, we need to reduce the deficit. You can't go round making promises like that."

He said Mr Clegg was making "easy" promises that the Tories would match but was failing to tackle the main issues.

"I think what you need to ask is: who's going to get on with this quickly, who's going to make an early start and who's telling you not the easy stuff - getting rid of ID cards, that's easy, getting rid of the contact database for children, that's easy, we can do that, get rid of the regional assemblies, that's easy.

"Ask yourself: who is telling you the difficult stuff? Who is saying freeze public pay for a year, who is saying retire a year later?"