The firm was the biggest winner among six contractors who all had their fines cut by the tribunal today saving more than £37m from an original £42m in penalties.

The six were among the first to launch appeals in the wake of an OFT bid-rigging probe which saw fines imposed of £130m against 103 contractors in 2009.

Another 19 contractors are awaiting appeal decisions which could see further cuts in fines.

The six successful appeal rulings today saw:

Kier’s fine of £17,894,438 reduced to £1,700,000.

Ballast Nedam’s fine of £8,333,116 reduced to £534,375.

Bowmer and Kirkland’s fine of £7,574,736 reduced to £1,524,000.

Corringway Conclusions’ fine of £769,592 reduced to £119,344.

Thomas Vale’s fine of £1,020,473 reduced to £171,000.

John Sisk’s fine of £6,191,627 reduced to £356,250.

The tribunal ruled that the original fines were excessive for the “simple” offence of cover pricing given that “the practice was long-standing in the industry and widely regarded as legitimate”.

The ruling also said that using 5% of turnover as a basis for calculating fines was too high.

The OFT is now considering taking the case to the Court of Appeal.

It said: “Today’s judgment was limited to the level of penalty for these six companies, who did not challenge our finding that they engaged in illegal cover pricing, in breach of competition law.

“The OFT will consider this judgment in detail, alongside those in the 19 other construction cases yet to be determined, and will consider whether it should appeal to the Court of Appeal.

“Financial penalties play a key role in deterring the companies involved, and other businesses, from breaching competition law. The OFT will consider whether the judgment has any implications for its practices and policies for ensuring a high level of compliance with competition law in the future.”