Britain's main safety regulator threw the government's energy plans into chaos tonight by damning the nuclear industry's leading designs for new plants. The Health and Safety Executive said it could not recommend plans for new reactors because of wide-ranging concerns about their safety.

The leading French and American reactors are central to plans for a nuclear renaissance aimed at keeping the lights on and helping to cut carbon emissions. The government needs to build a number of nuclear power stations in the next 10 years to replace old atomic and coal plants.

But the HSE has to approve the safety of the designs before they can be built. "We have identified a significant number of issues with the safety features of the design that would first have to be progressed. If these are not progressed satisfactorily then we would not issue a design acceptance confirmation," the agency concluded following a study of the latest French EPR and US AP1000 reactor designs.

Kevin Allars, director of new build at the HSE, admitted frustration that the design assessment process was already behind schedule owing to insufficient information from the companies promoting the reactors and a lack of enough trained staff in his own directorate.

The HSE's public report expresses "significant concerns" about the lack of separation between the safety protection and control systems on the EPR reactor design promoted by Areva and EDF of France. The safety body says another part of the reactor is "not entirely in alignment with international good practice".

The report says it has raised a number of issues with EDF and Areva relating to the structural integrity of the EPR and it concludes: "It is too early to say whether they can be resolved solely with additional safety case changes or whether they may result in design modifications being necessary."

The design put forward by Westinghouse, the American firm now owned by Toshiba of Japan, is also criticised, with the HSE saying the safety case on internal hazards has "significant shortfalls".

It criticises the company for a "lack of detailed claims and arguments" to support various assertions, while questioning aspects of the civil and mechanical engineering plans as well as the structural integrity and "human factors".

It also complains that the reactor design was submitted in feet and inches rather than metric figures.

Industry experts said the HSE was in a pivotal position to make or break the government's wider plans because it could delay the planned reactors from coming on stream from 2017.

That is the time that ministers fear an "energy crunch" because most existing reactors will have been retired, many coal plants shut down and renewable power insufficiently advanced to take over.

John Large, a leading nuclear consultant, said: "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."

Allars said he had not received any pressure so far from the government. While he had beefed up his staff and hoped to quicken the speed of his work, he insisted it was not his problem to worry ultimately about delays. "I am independent of government, and independent of industry and I will do what I need to protect society from any dangers of nuclear power. I will only be in a position to agree a generic design assessment if I get the right information [in future] to do that," he said.

The HSE said it might allow so-called exclusions over some of its concerns under which it would allow construction to proceed on the understanding that the problems would be addressed later.

Jean McSorley, consultant to Greenpeace's nuclear campaign, said it was highly likely reactor designs would not be ready for final sign-off at the end of the design process.

"This could leave the utilities and construction companies with real problems finishing projects, and that's very risky for them financially. Investment companies will also want to delay putting money into these projects until it is decided who takes responsibility for any potential cost overruns and delays," she said.

Areva shrugged off the concerns raised by the HSE. "It is important to emphasise that this is a normal part of what is a very transparent process and that it is entirely expected, as part of the design assessment process in the UK, for issues to be identified and resolved prior to licensing and construction," it said.