Staff at the Thames Barrier, one of the largest movable flood barriers in the world, could escalate their dispute over pay with the Environment Agency after three months of industrial action.

Unison, the public service union, said real-terms pay had consistently fallen over the past decade for its members working at the Thames Barrier in London, and so it was considering asking them to withdraw from incident response cover for a longer period in September.

However, this would fall short of strike action, as the workers would be completing all the normal hours in their contracts while refusing to take on non-mandatory additional jobs.

Members of Unison employed by the Environment Agency have already been withdrawing from incident response rosters on one or two days a week as part of efforts to force an improvement to their 2018 pay offer, which the union said gave an average 1.3% pay rise to those not working in nuclear teams.

Unison said this amounted to the 10th year of pay cuts. “This pay settlement imposed on staff was another slap in the face for staff who have been hit hard by a decade of austerity, and amounts to a real-terms pay cut,” said Unison’s national officer for water, environment and transport, Andrew Dobbie.

“Employees do not take action lightly but will not do work above and beyond what is in their contracts. The government and the agency must deliver a fair pay rise. Staff who work hard to maintain vital flood responses and protect the public are simply not prepared to accept another financial blow.”

The union said actions would be suspended if staff received what they deem to be a fair increase in pay from the Environment Agency.

The non-departmental public body has lost almost 1,000 members of staff who have been transferred to the Department for the Environment, Food and Rural Affairs (Defra), its sponsor, since July 2016 as part of efforts to save the department £124m. Unison suggested that cuts to the Environment Agency’s budget mean it may not be able to afford any prospective increase to staff salaries without a funding injection.

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The Thames Barrier covers 520 metres (1,700ft) across the river, protecting London from flooding in the event of surges in the tide.

More than half a million homes, the Houses of Parliament, Canary Wharf and 16 hospitals are among the attractions, transport infrastructure and institutions on an “at risk” list. There have been heightened fears of flooding across the country recently following extreme weather and heavy downpours.

An Environment Agency spokesman said: “We have robust arrangements to ensure that the Thames Barrier can continue to operate as necessary to protect London against the risk of flooding.”

Unison said that all its 3,000 members in the Environment Agency were involved in just working to their contract. There are reportedly 80 members of staff at the Thames Barrier.