Both sides say safety will not be affected during the strikes

British Nuclear Fuels Limited (BNFL), which operates the complex, has offered to go to arbitration to settle the current shift pay dispute.

Sellafield has already suffered two damaging one-day stoppages.

But union officials have said the offer of arbitration has come too late to halt Friday's walk out.

A spokesman for the GMB union, which represents many of the several hundred shift workers involved, said if progress is made next week, then further planned action may be suspended.

Harmonise differences

The industrial action is being taken by members of the GMB and Amicus unions.

It follows union claims that BNFL has not met an agreement to close a �2,000 gap in shift pay between industrial workers and staff.

Unions say BNFL made an agreement to harmonise the difference in shift pay between industrial workers and shift workers by April 2004.

But they say the company will not complete the process until April 2009.

The director of Sellafield, Brian Watson, said: "We did a lot of work with our trade union colleagues over a long period of time and believed we had reached an agreed position with the trade unions.

"That position was not accepted by the Sellafield trade unions and that unfortunately has put the whole negotiating process that we went through in some doubt."

Both sides have said safety at the plant will not be affected by the strike action.