A campaign by Unite comes as some workers were found to be earning less than £6-an-hour on sites.

The union is meeting with civils contractors and employment agencies today to encourage them to pay the London Living Wage which is set to rise to £9.15 an hour.

Firms will be warned that Unite will target companies who refuse to sign-up.

Unite construction national officer Bernard McAulay said: “This is something that needs to be addressed urgently and we will be leading the fight on this.

“We will also be talking to clients to impress on them the need for proper pay on sites in London.

“If companies don’t come on board with this then Unite has shown how successful its campaigns can be.”