Gordon Brown has met Charles Clarke to discuss the cost of national identity cards amid mounting concern from Labour MPs about the expense of the project.
The chancellor and home secretary met last week following talks between officials over detailed estimates of the cost of the scheme, which would initially be voluntary.
Ministers face a backbench rebellion by Labour MPs when the ID cards legislation has its second reading next week. While ministers are confident that the scheme, a manifesto commitment, is supported by the majority of backbenchers, they admit there is concern about how much taxpayers will be charged.
The Treasury denied suggestions in Whitehall that there had been a row between Mr Brown and Mr Clarke. But a senior government adviser said the chancellor had warned Mr Clarke he did not have a "blank cheque" for the scheme.
Another insider said: "The clear premise at the minute is this is financed either out of the Home Office budget or the fees [paid by those who apply for cards]. The Treasury is reminding the Home Office of that position."
The insider played down the implications of discussions between the departments on the issue, saying: "Shock horror! Treasury says watch the cost."
A Treasury official said: "As with any major public spending project, the Treasury works closely with the department involved on the costs to ensure maximum efficiency and taxpayer value."
While ministers are confident of getting the bill through parliament, there is nervousness in Whitehall that the project could become an expensive error.
In a sign of the sensitivities, the Home Office has decided not to disclose the full cost of applying for a card until after the bill setting up the scheme is law.
Rebel Labour backbenchers accuse ministers of hiding the true cost after learning that the charge for a card and any subsidies offered to the unemployed and the poor will not be set out until after the legislation has cleared parliament.
Ministers insist they cannot give details of the fees, and the amount companies will be charged to verify cardholders' data, until further work has been done once the bill has reached the statute book.
John McDonnell, a Labour MP, said: "The government is not releasing the figures because they know if they came clean on the actual costs our position will harden against the bill at the earliest stage.
"Every ministerial statement and every ministerial briefing is resulting in opposition hardening at each meeting."
The official estimate of the cost of the scheme over the next decade of £5.8bn equates to £93 per card. But ministers will not say how much of that will be recouped by charging those applying for cards.
In addition, by declining to give details of concessions, the government is leaving open the possibility that other people wanting ID cards will pay more to subsidise the less well-off.
