Fines of £39.27m were handed out to six firms in September 2009 following a probe by the Office of Fair Trading.

But those fines were cut to £14m today after the tribunal ruled that the turnover calculations used to assess the penalties were flawed.

Hays was the biggest winner as its fine was cut from £30,359,129 to £5,880,000.

Eden Brown Limited saw its fine fall from £1,072,069 to £477,750 and CDI AndersElite Limited had its penalty cut from £7,602,789 to £1,543,500.

The decision is the latest setback for the OFT which saw fines handed out to contractors in its bid rigging probe cut by the tribunal last month.

The OFT is considering an appeal against the latest decision.

A statement said: “Today’s judgment concerns the level of penalty for three companies, who did not pursue a challenge to the OFT’s finding that they breached competition law.

“The Tribunal upheld our view of the nature of the infringements, stating that this was ‘a very serious violation of competition law’.

“The OFT will consider the judgment in detail, including deciding whether to 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.”