A vote for strike action would see lines deserted

The Rail Maritime and Transport union will announce the result of a ballot on industrial action on 17 May.

It must give seven days' notice of any industrial action but the dispute over pay and pensions threatens chaos across the country before the month's end.

And it could last much longer, Mr Crowe told industry newspaper Railnews.

"We have to make it absolutely clear that we can't say when this dispute will be over," he said.

"People should go in with their eyes wide open."

Network Rail chief executive John Armitt, writing in Railnews, said the company would only be able to provide a skeleton service if RMT workers went on strike.

We stand to lose a lot of money Network Rail chief executive John Armitt

"Passengers will suffer a double whammy with the consequences of a strike and the efficiency of the network in the future," he added.

"We stand to lose a lot of money."

Mr Armitt said the company's offer of a one-year pay deal worth 3%, or a two-year agreement worth 3.5% in year one and inflation plus 0.75% in year two, was good.

But a union spokesman said Network Rail had "unilaterally imposed a cheaper and inferior pension scheme on new entrants without a word of negotiation".

"And it is not just an attack on new entrants, it is also an attack on the existing pension."

The company is out to save money by squeezing it out of its workers RMT

The union says the diminishing number of workers left paying into the final salary scheme will face ever-increasing contributions to maintain their benefits.

"We want the company to talk to us," the spokesman said.

"It is refusing to negotiate with us, and that is why we are forced into a ballot."

The RMT says the new scheme is no better than a "glorified savings plan".

Industrial action

"The company is out to save money by squeezing it out of its workers", the spokesman added.

"If it wants to talk to us about ensuring our members have a bit of security in retirement, it is never too late to call off a strike."

Transport Salaried Staffs Association members at Network Rail - mainly administrative staff - are being asked if they want to hold a ballot on industrial action, also over pay and pensions.

Meanwhile talks over the pay of London Underground workers will continue on Friday, with union leaders warning of a dispute unless a deal can be agreed soon.