Keltbray was among ten firms fined a total of £60m by the Competition and Markets Authority last March following an investigation into cover bidding in the sector.

Keltbray had entered into a settlement agreement with the CMA in which it admitted liability for eight infringements, and accepted that the CMA would impose a maximum total penalty of £20m to which a reduction of 20% would be applied in recognition of the procedural efficiencies achieved through settlement.

But it then launched an appeal against the £16m fine claiming it was excessive based on its turnover.

But the Competition Appeal Tribunal has ruled against Keltbray and increased its fine to £18m.

Juliette Enser, Executive Director of Competition Enforcement at the Competition and Markets Authority, said: “We are pleased that the Competition Appeal Tribunal has increased the penalty Keltbray has to pay from £16 million to £18 million for their part in illegal bid-rigging in the form of cover bidding.

“This judgment confirms that serious breaches of competition law, including for cover bidding, will result in significant penalties.

“The CAT agreed that, having appealed, Keltbray should lose the discount it received for settling. The CAT’s judgment confirms that companies will be held to their agreements – companies which settle cannot take the CMA to court and expect to retain their discounts.

“Today’s decision should act as a reminder that the CMA will not tolerate unlawful conduct which harms competition and can keep prices up at the expense of businesses and taxpayers.”