Consortia vying for the contract to decommission £7bn of nuclear waste are spending as much as £1m a month on bidding.
In a sign that government procurement costs remain eye-wateringly high, sources said the four bidders had spent "well into the teens of millions" on working up their proposals in a process that began 18 months ago.
The contract, formally launched last summer, will see the clean-up of 12 nuclear sites including obsolete magnox stations in Gloucestershire and Essex.
"There are so many different workforces, cultures and projects on these sites that you have to come up with different plans for each of them," added one of the sources. The sites are also at different stages of decommissioning, with one, in Anglesey, still producing electricity.
The costs are set to escalate further because the winner will not be announced until September 2014. At the moment, the short-listed teams are in what is known as "competitive dialogue", a controversial EU process that involves bidders revealing sensitive information to each other.
The bidders include a pairing of listed British engineers Amec and Atkins, while the remaining three teams are led by US giants Bechtel, CH2M Hill and Fluor. A fifth team, led by another US group in URS, recently withdrew from the auction. The contract will run for 14 years.
A spokesman for the Nuclear Decommissioning Authority, which will award the contract, said this is a "complex competition".Reuse content