The progress of the Government's nuclear power expansion plans was dealt a blow today after experts raised a raft of safety concerns about two proposed reactors.
Health and safety experts said they had identified a "significant" number of issues over the designs of the French and American reactors set to power a new generation of UK plants in the next decade.
In reports out today on the assessment of the French EPR and US AP1000 designs, the Health and Safety Executive (HSE) said there was a much more "detailed work" to do before they could be approved for use.
The HSE said of both units: "We have identified a significant number of issues with the safety features of ...that would first have to be progressed.
"If these are not progressed satisfactorily then we would not issue a design acceptance confirmation."
Among the criticisms raised, experts said there were significant concerns about EPR's proposed architecture, and that improvements were required for "hazard barriers".
Other issues relating to the reactor's structural integrity were also addressed, with the report saying it was "too early to say whether they could be resolved solely with additional safety case changes or whether they may result in design modifications being necessary".
The safety case of the AP1000's internal hazards also showed "significant shortfalls", it was found.
The HSE's assessment work is due to be completed within a year and a half, but one industry expert suggested a delay of up to three years was possible.
Kevin Allars, a nuclear assessment director at the HSE, highlighted shortages of information about the new reactors from their manufacturers.
He said: "Despite some initial resource shortages in some technical areas, we have made good progress on our assessment and we have identified issues that require further work."
"We are now at the stage of discussing these issues with the design companies, who are putting in place plans to resolve them.
"If we are to complete (design assessment) by the target date of June 2011, we will need the design companies to work with us proactively and to provide the high quality and timely information we need."
Ministers have announced plans for 10 new nuclear power stations to help offset the country's carbon emissions.
The first is set to be operational by 2018 and, by 2025, nuclear electricity generation could amount to around 40% of new energy provision.
The announcements were coupled with moves aimed at speeding up planning decisions on new energy projects aimed at cutting decisions to one year.
John Large, a leading nuclear consultant, aired the possibility of the HSE delaying the designs acceptance by up to three years.
He told The Guardian: "The HSE as an independent agency will come under tremendous pressure to push through these designs.
"But if it stands up to (the) government and stops or delays these designs for two or three years until it is satisfied then developers could lose interest and we could fall behind in the queue of countries waiting to build nuclear."