The NHS and other public services will have to find �1.2bn to fund the new penny increase in national insurance (NI), Downing Street has confirmed.

The measure, announced in last week's Budget, will mean some of the extra money promised for public services will in fact be clawed back.

The Conservatives have seized on the news, condemning the move as "a spectacular own goal".

The Treasury, in turn, said it had already accounted for the higher employer national insurance contributions (NICs) in the public sector and that it was the fairest way to raise revenue.

It is appropriate to target national insurance contributions and it is also appropriate that both employers and employees will have to pay Unison

Shadow Chancellor Michael Howard said the news made it clear the increase would hit "just those key workers who are vital to the success of Britain's public services - senior nurses, teachers and policemen".

"They are handing out money with one hand and taking it away with the other. It is a spectacular own goal."

They were already "leaving in droves because of low morale and lack of reform", he added.

Mr Howard's criticism echoes that of a hospital consultant, who accused Chancellor Gordon Brown and Prime Minister Tony Blair of scoring "an own goal" with the NI increase when they visited the Chelsea and Westminster Hospital the day after the Budget.

'Better health'

But a spokesman for Unison told BBC News the public services union supported the government.

"It is appropriate to target national insurance contributions and it is also appropriate that both employers and employees will have to pay," he said.

"It is by no means an own goal for the government.

"At long last we have a government that is willing to invest in the public sector.

"Business will benefit in the end because workers will be in better health."