The judge presiding over the continuing legal battle between Multiplex and Mott MacDonald over Wembley stadium is urging the pair to resolve it by mediation, as he revealed that combined costs for the firms have risen to £73.2m.

Multiplex is expected to spend £45.7m on the case by next January (of which £12m has already been spent on expert advice); Mott MacDonald’s costs will reach £27.5m.

It also emerged that the £253m case, which is due to be heard in January 2011, would now take four weeks instead of the envisaged two, leading to rising costs on both sides.

In a hearing this week, Mr Justice Coulson said a mediation service “would be a sensible next step, to be taken sooner rather than later”.

He added that should the case be heard, he would base his cost orders in part on both firms’ willingness to participate in mediation.

A legal source said: “These costs are staggering. The firms need to seriously consider mediation, otherwise whoever wins could still have costs awarded against them.”

Meanwhile, Mott MacDonald is continuing to prepare its counter-defence, which will come in one month later than expected, at the end of April.