The Enquirer understands that construction union UCATT is encouraging workers to down tools as part of a new long-term campaign against umbrella companies.

It also signals a switch in tactics by the union as it prepares to merge with Unite which is known for a more aggressive industrial relations strategy.

A UCATT insider, said: “We are going for a much for more industrial campaign – this is not about protesting outside sites this is about workers taking action which will hit contractors in the pocket.

“Workers are sick and tired of paying umbrella companies and this campaign will show them that by undertaking a bit of collective action they can do something about it.”

The first action is expected on sites in London and Scotland this week.

Umbrella companies are used in construction as intermediaries to maintain a workers self-employed status.

They have come under fire for charging high fees and leaving workers to pay all National Insurance contributions.

The source added: “This is all about putting pressure on contractors and clients to stop using umbrella companies.

“There’s been a lot of talk about stamping them out and now is the time to get action going on site to make it finally happen.”