Tories and Liberal Democrats claimed M&C Saatchi's TV, radio and press blitz had added to the public's confusion about the complex new cash support available to families which took effect this month.
Matthew Taylor, the Treasury spokesman for the Liberal Democrats, said that despite the huge campaign, the Government had failed to inform people that they needed to claim by the end of January. "The advertising material - the main warning of what people had to do - did not feature that information with any prominence," he said.
David Willetts, the Tories' shadow minister for work and pensions, said: "The Government tried to use the advertising campaign to cover up a fundamental design fault in the system. The whole thing has been a shambles."
But Dawn Primarolo, the Treasury minister, backed calls by Labour MPs for M&C Saatchi's campaign for the Inland Revenue to be extended to boost the take-up of the new benefit.
"Advertising will continue, to remind those who do not know about the tax credits that if they put in their claim now, it can be backdated to 6 April," she said.
Primarolo, who denied the system was in chaos, said that so far £8 million had been spent on TV ads, £2.2 million on press, £876,000 on radio and £400,000 on online. Some £95,000 was spent on design, print and production of promotional materials.