A row over pensions could spark industrial action by tens of thousands of Tube and other transport workers in London, a leading union warned today.

The Rail Maritime and Transport union said employers with a stake in the Transport for London Pension Fund will be in dispute from tomorrow unless they give an assurance that they will not support changes the union claims will "dramatically affect" qualification for ill-health pensions.

The scheme has more than 22,000 members working for London Underground and infrastructure companies including Metronet and Tube Lines.

General secretary Bob Crow said: "At present, ill-health pensions are granted to workers in the TfL fund whose ill-health makes them unfit to do their particular job.

"Proposed changes could mean that anyone capable of earning an income, regardless of how small, would probably not qualify for or retain an ill-health pension.

"That is a fundamental attack on a long-standing condition of service that would see most people who leave work through ill-health deprived of a pension, and we will resist it.

"We have warned the various employers that we expect an unequivocal commitment that they will not seek to undermine our members' pension rights in this way, or we will be in dispute with them."