The four-day walkout will take place after talks at the conciliation service Acas failed to reach a resolution.

A four-day strike by workers on London’s Docklands Light Railway is to go ahead after talks failed to resolve a dispute over outsourcing.

Members of the Rail, Maritime and Transport (RMT) union will walk out from 4am on April 20, covering the weekend of the London Marathon.

Talks were held at the conciliation service Acas but ended without agreement.

RMT general secretary Mick Cash said: “RMT continues to push for a negotiated settlement in a dispute that is about turning the tide on outsourcing and ensuring workplace justice for this key group of London transport workers.

“Our negotiating team is frustrated that the talks have failed to make any significant progress on the key issues.

“RMT has made it absolutely clear that this dispute can be resolved and that there is still time to reach a settlement but that requires a display of the same kind of will and commitment from the management side to move the issues forwards.”

The union served notice on Keolis Amey Docklands, which operates the DLR, for the next phase of strike action, saying it was in dispute over outsourcing and abuse of procedures.

A 48-hour strike was held last month, causing travel disruption for thousands of passengers.

Huge crowds are expected to line the route of the London Marathon on April 22.

Mark Davis, Transport for London’s interim general manager of the DLR, said: “Millions of pounds are raised for charity at the London Marathon and we don’t want to see the thousands of people who have signed up to run, or their supporters, disrupted.

“We urge RMT and Keolis Amey Docklands to continue with discussions to settle their dispute. If the strike goes ahead, we will work with the operator to provide as much of a service as possible. We will ensure customers have all the information they need to re-plan their journeys during this unnecessary strike.”