The Tories have accused the chancellor, Gordon Brown, of blatant party propaganda by running M&C Saatchi's £5 million blitz just three months before the general election expected in May.
The Opposition has asked top civil servants to pull the TV, poster and press work. But they are likely to continue, because Whitehall rules say government campaigns should be halted only when an election is formally called.
George Osborne, the shadow chief treasury secretary, said: "The Government is spending a small fortune advertising baby bonds. This smacks of a blatant attempt at a pre-election bribe."
He argued the campaign is unnecessary because parents will receive a voucher in the post. The Treasury argues the ads are needed so that people do not ignore the letters.
Osborne has written to Gus O'Donnell, the Treasury's permanent secretary, urging him to ensure that "no political impropriety" takes place.
O'Donnell replied that the Inland Revenue's campaign was needed to "raise awareness among parents". He added: "If a general election were to be called, the Government will, of course, fully comply with all the usual rules on the handling of information."
The Government is providing £250 for each child born after September 2002, with £500 for children from the poorest families.