The public body in charge of Sellafield has tried to conceal evidence that it seriously underestimated the spiralling costs and delays of the biggest nuclear construction project in the UK.

The Nuclear Decommissioning Authority (NDA), which is the legal owner of the reprocessing site in Cumbria, denied the original estimate for the "evaporator D" project was £90m when the The Independent revealed the true cost to be £400m or more in February.

In a statement released at the time, the NDA said: "The original plan of an Evaporator D to be delivered by 2010 for £90m has never been accepted by NDA as credible and we have no ownership for those numbers."

However, the minutes of a 2008 NDA board meeting show nuclear executives had originally estimated the evaporator D's costs to be about £100m.

The minutes state: "In discussion, the substantial increase in costs from the original £100m to the current £360m was observed... and the project was about four years behind schedule."

The minutes, which have been removed from the NDA's website, also reveal just how fast the costs of evaporator D were rising.

Another set of minutes, approved just six months earlier, said the costs had risen from £160m to £360m.

Yet in December 2008, the board was asked to sanction between £360m and £380m for the project.

Six months later, in June 2009, the estimated cost had spiralled significantly to £397m – the figure the NDA claims was the original estimate for a "fully scoped and sanctioned" project.

Michael Meacher MP, a former environment minister, yesterday condemned the NDA for trying to conceal the original estimates used to justify the taxpayer-funded project.

"It's a shifty and deceptive industry that is totally untrustworthy. This happens so regularly it must be a deliberate ploy to minimise costs knowing once a project is underway no one can do anything about it," he said.

Evaporator D is currently the single biggest nuclear construction project in Britain and involves the assembly of a building the size of an office block from 11 transportable modules manufactured on Merseyside and shipped to the site in Cumbria on a purpose-built barge.

The aim of the giant evaporator is to reduce the volume of the high-level liquid waste from Sellafield's reprocessing operations. The original delivery deadline of 2010 has now slipped to 2014, but on-going construction problems might mean that it is not ready even then.

The NDA, which took over legal guardianship of the Sellafield site from BNFL in 2005, has ordered a major internal review of the project because of the "disappointing performance" of the contractors carrying out the work.

The Office of Nuclear Regulation, which is the industry watchdog, said last year it does not have the full confidence in the ability of Sellafield Ltd, which manages the nuclear site, to complete the project on time.

The NDA said that it routinely removes the minutes of board meeting from its website after a 12-month period and the December 2008 board meeting was the first time a full financial assessment of the project was received.

"As described in the minutes the NDA then moved forward to the official sanction of £397m in July 2009," said an NDA spokesman.

"The work programme to review the project is not yet completed. We expect to make further announcements on the future delivery of this project during the summer."