London Underground workers are to be balloted on strike action in a row related to the firefighters' dispute, the Rail Maritime and Transport union has announced.

For the second day, about 80 Tube drivers have refused to work due to safety concerns during the 48-hour firefighters' strike, which is due to end at 1800 GMT on Friday.

The RMT rail union will ballot 7,000 members after fears some could be victimised for refusing to work.

The union wrote to the company, demanding an assurance that members would not be disciplined for refusing to work.

Some Tube stations closed over evacuation fears

No reply was received by the union's deadline of 1130 GMT on Friday.

General secretary Bob Crow said it was "outrageous" the government had been telling people they could not stay away because they considered conditions to be unsafe.

The union claimed Tube workers were being intimidated by managers and threatened with disciplinary action.

London Underground says it is meeting the drivers and will address any valid safety concerns.

'Unacceptable'

But the company said it would not be driven by the union's "inappropriate, artificial deadlines", and would reply to the demand for assurances in due course.

The action by drivers compounded misery for Tube travellers who already faced the closure of 22 stations because of the industrial action.

The drivers who stayed away were roundly condemned by Deputy Prime Minister John Prescott.

He said the Health and Safety Executive had judged it safe to run the services and added the drivers' action "cannot be condoned and is unacceptable".