Officials from the union Unison are going to ballot office and technical workers about plans to change their employment conditions.

Operator of the system, Nexus, wants to introduce a new, non-compulsory, scheme for 450 white collar staff.

Staff do not have to take up the scheme, but Unison says the new conditions would mean workers would be worse off in the long-term.

It says Nexus would be able to vary employees' conditions whenever it wanted to.

Nexus also wants to extend its random drug and alcohol testing programme to all its 1,000 staff.

'Way forward'

A Unison spokesman said they had not been consulted on the proposals, and that performance-related pay had not been shown to work in other organisations.

A spokesman for Nexus said they believed performance-related pay was "the way forward".

If Unison members vote in favour of strike action, it would take place within the next couple of months.

In autumn 2002 there was a threat of a six-day strike on the Metro by Unison members over pay for control room staff. It was called off when agreement was reached.