Tube workers may go on strike over plans to introduce staff identity cards, unions have said.
Up to 8,000 workers would be issued the cards
Proposals could see up to 8,000 Tube workers, including engineers and maintenance staff, issued with cards.
The Rail Maritime and Transport (RMT) union said the new system could be used to check hours worked by staff.
Metronet and Tube Lines, which maintains the network, said the card would be used to check workers' licences and would improve safety.
The RMT's Bobby Law warned that if the plans went ahead without agreement there would be a ballot for industrial action.
The union fears the card would include fingerprint recognition and could be used to discipline staff in disputes about clocking on and clocking off times.
"It is a gross infringement of human rights to use such a card to clock on and we will resist it," he said.
Metronet said talks were continuing and no decision had been made as to whether the card would include any biometric features.
"It is an option but it has not yet been agreed," said a spokesman.
Union officials said introduction of the new card was being linked to a new pay deal.