Only Gordon Brown could have invented a so-called tax cut so complicated that you have to fill in a form to apply for it

David Willetts

But the Conservatives say the claim process is so complicated the government has been forced to launch the advertising blitz out of "desperation".

And the Liberal Democrats say despite launching the credit two years ago more than a million eligible families still have not applied for the money.

Mr Brown wants to spread the net wider with the new campaign which he launched at a London nursery near his 11 Downing Street home.

A few of the youngsters were tongue-tied during his 15-minute stay at the nursery in Pimlico but the visit did not faze Grace Allan, three, who allowed the chancellor to play with her building bricks.

At a news conference later on Monday Mr Brown is expected to say it is one element in a parcel of targeted tax cuts that will help "parents to balance work and family responsibilities, and ensure all parents take responsibility for their children".

"Record increases in Child Benefit help all families, whilst the introduction of the Working Families Tax Credit, and now the Children's Tax Credit do even more for almost nine out of 10 taxpaying families," he is due to say.

Mr Willetts says government is 'desperate'

But shadow social security secretary David Willetts has warned that one million of the five million families supposed to benefit from the credit will miss out because the complicated rules for claiming it have put them off.

"Only Gordon Brown could have invented a so-called tax cut so complicated that you have to fill in a form to apply for it," he said.

"The advertising campaign is a sign of their desperation that millions of families that are supposed to get the Children's Tax Credit have not filled in the claim forms."

Liberal Democrat social security spokesman Steve Webb said a "bureaucratic nightmare" meant millions of families could miss out on the money they are entitled to.

"The government announced its plans to bring in this credit nearly two years ago, yet it has still failed to find over a million families.

"The government could have found every single one of these children had it wanted.

Labour's pre-election tax bribe strategy is in tatters

Steve Webb

"The Benefits Agency has the name and address of every child in the land. But they are so embarrassed that they are paying mainly fathers rather than mothers through the benefits system, that they are prepared to spend millions of taxpayers' money on publicity campaigns.

Mr Brown's latest promotional push follows his declaration of a war on poverty with a raft of new tax and benefit measures to be introduced in Labour's general election manifesto.

Reducing child poverty, raising incomes for pensioners, and reforming the health service are the main priorities.