Maintenance workers will be balloted from 14 March

The Rail Maritime and Transport (RMT) union said maintenance firm Metronet planned to break an earlier agreement by transferring workers to other firms.

The union said that if strikes went ahead, more than two-thirds of the Tube system would be crippled.

Metronet said industrial action would be "unjustified and unwarranted" as its discussions with the RMT were ongoing.

Metronet is the consortium responsible for maintaining and upgrading parts of the Tube.

'Maximising profits'

The union said Metronet previously agreed not to transfer employees without agreement with the RMT, following a similar strike ballot last year.

RMT general secretary Bob Crow said: "Forcing our members to transfer is about maximising profits and dividing and undermining our members' organisation, pay and conditions.

Unions should act responsibly and not resort to threatening strike action at the drop of a hat Geoff Pope, London Assembly

"It also means forcing even more fragmentation on a network that has already seen its safety culture undermined by the disastrous Public Private Partnership."

Metronet said that "as part of its contractual obligations" it had originally planned to transfer more than 1,000 staff to Bombardier - one of four companies in the consortium.

"However since 2005 Metronet has been looking at a compromise agreement that will transfer just 250 staff, primarily managerial and technical specialists," a spokesman said on Friday.

"We would maintain the vast majority of staff within Metronet."

'Act responsibly'

London Assembly Transport Committee chairman Geoff Pope said he was surprised that a strike ballot was planned, after a separate pay dispute was resolved between the RMT and London Underground on 28 February.

"Just over a week ago the mayor heralded a new era of stability for industrial relations on the Underground," Mr Pope said.

"It beggars belief that eight days later Londoners are, once again, threatened with strike.

"Unions should act responsibly and not resort to threatening strike action at the drop of a hat."