The contractor is facing a £60m claim from Cambridgeshire County Council for delays and cost overruns on £151m project.
But Bam alleges that consultant Atkins, which was project manager on the job, was to blame for much of the delay in its counter claim.
The busway project was due to be finished in early 2009 at a cost of £116m. It was finally handed over to the council a year ago and is reported to have cost £152m.
In papers lodged at the Technology and Construction Court, the company said: “Through out the course of the works, the project manager failed to act independently and fairly and/or to act as stated in the contract in the spirit of mutual trust and co-operation.”
Atkins strongly denied the allegations. Its spokesman said: “We were engaged as the independent administrator on the Cambridge busway scheme and we are confident we always acted in the best interests of the project.
“We are unable to discuss any details at this stage due to the ongoing legal action.”
The council’s claim stems from alleged defects, which meant it refused to accept handover of the project on several occasions. During this time the council charged Bam Nuttall £14,000 a day for late delivery.
Bam Nuttall claims it completed the busway in December 2010, but the council did not accept it until May 2011. As a result, the company claims it is due an extra £43m.
Bob Menzies, the council’s head of infrastructure delivery, said: “We are pleased that, after a number of delays, Bam have provided their defence and counterclaim to the legal claim submitted by Cambridgeshire County Council in August of last year.
“Bam’s documents will now be reviewed in detail by the council’s legal team. The council has obtained extensive advice on the busway contract and remains confident of its position.”
A trial date has been set for January 2014, and the council has set aside £6.5m for legal costs.