The move by the M&E giants has already sparked uproar among site workers who fear pay rates could be cut by up to 35% as new grades are introduced.

Electricians staged their first demonstration outside Balfour Beatty’s Blackfriars station job yesterday.

Any doubts from the clients will be a major blow to the breakaway eight who are – Bailey Building Services, Balfour Beatty Engineering Services, T. Clarke, Crown House Technologies, Gratte Brothers., MJN Colston, SES and SPIE Matthew Hall.

Clients are understood to be nervous about the spread of rank-and-file demonstrations and possible disruption to major projects.

A Unite statement said: “We have been informed by a few major industry clients that they do not support this breakaway move.

“They are concerned that industrial relations unrest caused by these proposals could jeopardise the ability of companies to deliver projects on time and within budget and that harmonious industrial relations will be a serious consideration for these clients when awarding future contracts.”

The breakaway is also opposed by a number of trade bodies including the ECA and SELECT, and plumbing groups APHC and SNIPEF.

The eight rebel firms have given notice to quit the JIB agreement next March.

One contractor said: “A change is long overdue and most workers realise that our proposals are a sensible way forward.

“The opposition is the usual bunch of hotheads and we will not be held to ransom by threats from a very small minority.”