Bonuses were withheld following Grayrigg the crash in February

The Rail Maritime and Transport Union (RMT) is accusing Network Rail of refusing to negotiate settlements in disputes involving 500 staff.

The disputes involve 400 signallers in Scotland as well as 100 RMT members in Cumbria, whose bonuses were withheld following a crash in Grayrigg.

Network Rail said the RMT's approach was "wholly unnecessary".

Both sets of workers are being balloted for industrial action, with the results due to be announced at the RMT's annual conference in Edinburgh next week.

'Double standards'

The Scottish workers had their bonuses withheld following strike action in March.

RMT general secretary Bob Crow said: "Emergency motions coming into our AGM are calling for a national strike ballot, and it is clear that there is a head of steam rising over Network Rail's double standards and intransigence.

"I have been in contact with Network Rail seeking talks, but the company has told us flatly that the bonus issue is non-negotiable.

"As a result, it is now looking likely that we are heading towards a national stoppage in the next four to five weeks."

A spokeswoman from Network Rail said: "We are disappointed that the RMT is taking this approach, which is so wholly unnecessary and will only cause disruption to passengers.

"Those who took strike action in Scotland had a clear understanding at the time that their discretionary bonus would be at risk, so it can hardly be surprising that their bonuses were affected.

"It would also be inappropriate to pay a bonus to the maintenance workers in Lancashire and Cumbria before the investigation into what happened at Grayrigg has been completed."