Workers held a two-day walk-out before Christmas

On Tuesday, employers Laing O'Rourke offered scaffolders and labourers an extra 67p an hour in bonus pay - short of union members' call for £1 an hour.

But GMB and Transport and General Workers' Union said until members are balloted, the strike will take place.

Hundreds are expected to walk out on Friday and Monday for the second time.

Workplace ballot

The unions said the offer had not been received in writing and they had not been given permission to hold a workplace ballot.

GMB regional officer Phil Davies said: "In the meantime, as far as we are concerned and until there is a ballot accepting the offer, the industrial action will take place."

Laing O'Rourke earlier said it expected the new offer to be accepted as it would confirm Terminal 5 workers as enjoying the best terms in the industry.

In December, workers staged a two-day walk out in the row over bonus pay.