Further protests also took place at major sites in Manchester, Newcastle and Grangemouth over plans by the major eight  building services contractors to ditch the industry wage agreement.

Campaigners claimed electricians and pipe-fitters working for Balfour Beatty Engineering Services at Grangemouth in Scotland downed tools and walked off the job this morning.

Further action also took place at Balfour Beatty’s Papermill site in Manchester

At the Olympics site in London police were forced to intervene after 150 electricians blocked the main entrance causing rush hour travel chaos with vehicles backing up around the A12 Bow Flyover.

One of the protest organisers, Alan Keys, said: “It’s not the Polish or Portuguese workers who are cutting our wages, but the industry bosses.

“They can threaten to sack us and blacklist our stewards but if they think we are going to just sit back and accept this, they are very much mistaken.”

Electrical contractors argue that a new pay grade must be introduced for installers, but have stressed the reform will not affect the pay of existing staff.

A spokesman for the contractors said: “We will not be regrading anybody or changing anyone’s pay rates on site. The new installer grade will only be applied to people joining the industry.”

Balfour Beatty, Crown House Technologies, Spie Matthew Hall, Shepherd Engineering Services and NG Bailey have told the union Unite that workers must sign new deals by December 7 or face the sack.

The last time the electrical contractors attempted to reform wages was 12 years ago.

This provoked a series of coordinated strikes on the Jubilee Line, Royal Opera House, Pfizers and several power station projects.