A critical report published today from the National Audit Office warns that six of the seven major construction projects at the huge site are behind schedule.

A long-term plan to clean up the Nuclear Decommissioning Authority-owned site was agreed last year after an earlier one stalled because it was “unrealistic”.

The NAO report found that after only 10 months of the new plan, performance in 12 of the 14 big buildings and equipment projects considered “critical” for reducing risk had been “dire”.

During this short time frame six of the seven major projects under construction accumulated delays of between two and 19 months.

Estimates for the total cost of these projects increased during this period by £294m with most of the cost escalation in the evaporator D project.

The report warns of gaps in the capability of Sellafield subcontractors and construction projects starting before design risks were sufficiently addressed.

The Authority estimates that £50m of the £244m increase in the cost of evaporator D and part of the 18-month delay since 2009 was down to the subcontractor lacking experience in welding to the necessary nuclear quality standards.

Margaret Hodge, who chairs the public accounts committee, said: “Projects of this length and ambition are ripe for dithering and delay.

“I am dismayed to discover the clean-up of Sellafield is no different. The authority’s revised plan sees critical milestones shunted back by up to seven years.

“My concern is that unless the authority holds Sellafield Limited to a clear and rigorously benchmarked plan, timetables will continue to slip and costs spiral.”

Around 240 of Sellafield’s 1,400 buildings are nuclear facilities and so far 55 buildings on the site have been decommissioned.