The threat of industrial action on London Underground increased when a leading union set a deadline for assurances about jobs, safety and working conditions in a growing crisis over finance.
The Rail Maritime and Transport union gave maintenance giant Tube Lines until Thursday to come forward with guarantees that there will be no cuts despite a funding row over the cost of upgrading the Tube.
General secretary Bob Crow said: "We have written to Tube Lines giving them until Thursday to give us concrete assurances that they will pull back from attacks on jobs, safety and working conditions as a result of the £1.3 billion row with the mayor.
"Essential Tube works, and the jobs and conditions of our members, should not end up as the collateral damage in the war over the failed Tube privatisation experiment."
The union made clear it will be in dispute if it does not receive assurances from Tube Lines, which is responsible for maintaining the Jubilee, Northern and Piccadilly lines.
It has also warned of a dispute over plans to axe up to 800 jobs on London Underground, mainly among station staff.
Mr Crow said the cuts will turn London's Tube stations into a "muggers' paradise", but London Underground insists there will still be staff on stations and at ticket offices.
The company said there would be no compulsory redundancies because of the cuts, announced following increased use of pre-paid Oyster cards.