Most of the £400m rise in the bill for identity cards will be spent ensuring the scheme has enough staff to be a success, according to Identity and Passport Service chief executive James Hall.

The bi-annual cost report published last week put a price tag of £5.3bn on the plan, up on the £4.9bn estimate last autumn.

But Hall says the previous calculations of the number of people needed to roll out the cards to all UK adults were unrealistic.

‘We have taken a more conservative view of the number of application and enrolment examiners needed to successfully deliver the scheme,’ said Hall.

‘We have factored in increased numbers in the short term and have avoided planning for reductions until we are fully transitioned. It risks the integrity of the scheme to reduce staff numbers at the same time as introducing new technology,’ he said.

The price tag will continue to fluctuate because of inflation and take-up levels, but costs are not expected to rise so sharply again.

‘With some caveats, and the fact that we have not yet started the procurement, we believe we have bottomed out the costs and do not see any reason for them to change significantly,’ said Hall.

He is also keen to quash speculation that Gordon Brown will cancel the scheme if he becomes prime minister.

The ID card plan has already been reviewed when John Reid became home secretary. And Hall says it is a cornerstone of Sir David Varney’s report on public service delivery, commissioned by the chancellor.

It is too early to criticise cost overruns because the project has not even started, says Eric Woods, director at analyst Ovum.