Airedale went into administration in July with the loss of 135 jobs. Its electrical contracting arm went under two months earlier in May.

The £66m turnover Leeds-based firm was working at sites across the country including BAM’s £25m Swiss Cottage SEN (special educational needs) School.

The school was due to open this month but will be three weeks late with its new completion date of October 1.

A BAM spokesman said: “BAM’s priority is to hand over buildings which are properly ready for occupation, and fully safe for pupils, parents and staff. We are working intensively to do this.

“The only reason this has not been possible as originally planned is the substantial problems the project has suffered as a result of major sub-contractor, Airedale, going into receivership at a critical phase of the programme, leaving an enormous range of mechanical and electrical problems behind.

“Until this happened in May the schools were on programme.

“Since then we have done everything in our power to put right these problems which were numerous, severe and unforeseen and we have been, and are still, working intensively to complete the schools as these problems have been discovered.

“At its own cost, BAM has had to re-employ workers, buy-out materials and bring in additional resources and has worked around the clock to offset the effects and get the schools ready for children to use and enjoy.

“As a responsible and experienced contractor, BAM has kept Camden Council fully informed about this issue throughout as we have encountered the problems left by losing such an important sub-contractor at a key stage, and communicated with the Council at the highest level.

“We deeply regret the unavoidable impact this will have for a few weeks on local people and their children.

“We had proposed a sooner date for occupation than has been adopted but we are deeply sad that there are any delays at all.

“The facilities that have been created and that will be handed over very soon will be a first-class learning environment for children which we know will enhance their lives.”

BAM has come under fire from Camden council chiefs over the delay.

Councillor Angela Mason, Cabinet member for children, Camden Council said: “I am deeply disappointed.

“We had written assurances from the highest level within BAM as recently as 20 August that the opening of the specialist SEN school would be on schedule.”

Balfour Beatty has also apologised for delays in opening its £40million new build project at Noel-Baker School and St Martins School in Derby.

Construction work has overrun and the school will now be handed over a week late next Monday