London Underground workers are being balloted on strike action in a dispute over safety, threatening chaos for millions of passengers.

The Rail Maritime and Transport Union (RMT) said it expected its 9,000 members on the tube to back industrial action in a dispute over the safety of a privatised network.

Signalling workers, station staff and drivers belonging to the RMT could stage a series of one-day strikes from early July if there is a yes vote for action.

Ballot papers go out on Friday and voting will end on 24 June.

'Defend safety'

RMT General Secretary Bob Crow said all tube unions were concerned that the proposed public private partnership (PPP) was unsafe.

"London Underground is rushing through its safety case and our members are saying clearly that they want us to defend their safety," said Mr Crow.

"We believe our members will back the call for strike action."

More than 2,000 workers will transfer to private firms under the PPP plans.

Mr Crow claimed that London Underground was being secretive about the timing of the PPP process.

He also accused the company of refusing to consult with unions about issues such as safety.

The RMT has written to the new Transport Secretary Alistair Darling, calling on him to intervene in the dispute.

A London Underground spokeswoman said the company was asking the RMT to discuss the issue so that any strike action could be averted.

"We have had extensive consultation over the last three years about PPP with the RMT and included them in the process. Claims that we haven't consulted widely is not right," she said.