The scan would check the employee against a federal database to confirm eligibility to work.

WSJ: Senators in Immigration Talks Mull IDs for All Workers

It has already been forced to eliminate two companies that had been chosen for the work because of questions over their eligibility.

WSJ: NY's Fire Island debris effort faces bird deadline

The BBC's chief political correspondent, Norman Smith, said Treasury officials were trying to work out a definition of the rules surrounding eligibility for the mortgage guarantee scheme that allowed parents to buy a home for their children but not to buy a holiday home.

BBC: Budget 2013: Row over 'second home subsidy'