British firm De La Rue is preparing to issue a legal challenge to the preliminary decision to award the contract to make the blue-coloured passports to Paris-based Gemalto.
Meanwhile, a petition run by the Daily Mail newspaper to have the non-EU passports made in Britain has topped 266,000 signatures.
The British government confirmed that a two-week extension to the bidding process began on Tuesday, after the Home Office interior ministry agreed to a request from De La Rue.
A spokesman said that such a “standstill” period was not uncommon.
He made clear that Gemalto remained the favourites.
“The preferred bidder demonstrated they were best able to meet the needs of our passport service, with a high-quality and secure product and best value for the taxpayer,” he said.
The return of blue British passports in place of standard burgundy European Union ones is seen as a hugely symbolic reclamation of UK sovereignty for those backing Britain's exit from the bloc.
Brexit supporters have said it would be a “national humiliation” if they were made in the EU.
De La Rue admitted it would be a more expensive option for taxpayers but insisted theirs was the “highest quality and technically most secure bid”.
“We confirm that we are taking the first steps towards initiating appeal proceedings against the provisional decision,” a spokeswoman added.
UK passports had dark blue covers from 1921. But Britain switched to burgundy from 1988, in common with other passports of what was then the European Community.
Britain is set to leave the EU in March 2019. The new passport production contract starts in October 2019.
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