All six sites of National Museums Liverpool will be picketed by Prospect members for the third time in two months. More than 250 conservators, archivists, administration and other specialist staff will take action, following their highly successful strikes on 25 July and 22 August.

The staff are fighting an imposed pay offer worth 1.8% for 2007 and 1.3% for 2008, less than half the rate of inflation, and have won growing public support for their campaign.

Councillors, NML trustees and MPs have joined hundreds of public visitors to the museum in expressing their concern at the continuing dispute and have called on NML to re-enter pay talks.

But Prospect has yet to receive any indication from management that it is willing to reconsider its position, said Prospect negotiator Tony Hammond. "The museum’s ‘act tough’ attitude has backfired. Meetings have been extremely well-attended and members are more determined than ever to keep the campaign going."

Staff have been angered by the museum’s response to MPs and trustees who have written to the museum. NML claimed it was the unions who had broken off the talks on pay when the reverse was the case, said Hammond. "NML imposed the offer before the membership ballot had even been completed," he pointed out.

Union representatives have written to all the city’s MPs seeking support, said Hammond. "We have had a great response – several have written to the museum and the number who have signed the Commons early day motion calling on NML to enter talks has risen to 18."

The Labour group on Liverpool City Council have carried a motion condemning NML’s low pay policy and Labour group leader Joe Anderson will be visiting the picket line to lend the group’s backing.

The six NML sites that will be picketed on 19 September are: The Walker Art Gallery, World Museum Liverpool, National Conservation Centre, the Maritime Museum, Sudley House, and The Lady Lever Art Gallery (Wirral).