Siegel added that the workers had no employment rights since they had taken unofficial strike action.
Northern Rail said it would seek to minimise disruption for customers if strike action went ahead.
We will update customers on strike action as soon as we have any further information.
The strike action was the first of its kind at the company in 20 years.
He said members had not been balloted for strike action, so any participation would be illegal.
The Communication Workers Union said 88% of its members had voted in favour of strike action.
The firm reported its first loss last year following the disruption caused by strike action.
Production at the Rustenburg mine was disrupted for eight weeks in 2012 following illegal strike action.
Stephen Twigg, Labour's shadow education secretary urged more negotiation in order to avoid strike action.
Lots of crew will be stuck around the world because of the strike action.
Taxi driver Imran Zaman said the strike action would continue indefinitely until there was a result.
Of the 300 who took part in the RMT ballot, 85% voted for strike action.
Strike action was averted two weeks ago when the company delayed introducing changes to working conditions.
But Remploy said strike action would do nothing to secure the future jobs of employees.
Bus workers in Jersey have threatened further strike action over changes to terms and conditions.
"We are in danger of witnessing strike action and consultants seeking employment elsewhere, " he said.
BBC: NEWS | UK | Wales | North East Wales | Consultants' ultimatum over pay
The PCS said 57.2% of those who voted had backed strike action - the turnout was 20%.
Ambulance workers in Yorkshire are to be balloted over strike action in a row over staffing changes.
Unison said administrative and clerical staff last month voted 88% in favour of the one-day strike action.
Eurostar said the strike action would involve around 160 customer services staff and would have no impact.
In response, Royal Mail said it had been working on contingency plans should strike action be held.
Railway workers have voted in favour of strike action over the use of casual staff by Northern Rail.
She said two further days of strike action were planned for May if a resolution was not found.
Unite said more than 650 of its members at Dagenham would vote on strike action in coming weeks.
Unison recently balloted its members regarding strike action, after a 5.4% pay cut was imposed on council staff.
Plants at Halewood, Southampton and Dagenham are among seven centres that will be affected by the strike action.
He said the union would not be rushing in to strike action, but would seek a negotiated settlement.
Disneyland's management attempted to play down the strike action, claiming that the actors were gradually returning to work.
But Conservative Party chairman Baroness Warsi said the strike action across the UK was "going to benefit no-one".
BBC: Public sector strikes: MoD picket outside Donnington base
The dispute has seen signallers in the Stirling area take strike action on several occasions, including Christmas Eve.
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