A second public consultation into whether new nuclear stations should be built has "failed", a report suggests.
The government has said nuclear should be part of the energy "mix"
The Nuclear Consultation Working Group said key issues had not been addressed "in any meaningful way" and it had been designed to "provide limited answers".
Greenpeace, which won a court challenge to the first consultation, says its lawyers are studying the report.
The government, which will publish the latest consultation's findings next week, said it had been "fair and full".
Most existing British nuclear power stations are due to close by 2023 and the government has said its "preliminary view" is that new stations should be built as part of Britain's "energy mix" - to reduce carbon emissions and reliance on foreign oil and gas imports.
'Hostility and mistrust'
Its first consultation on the matter was ruled "seriously flawed" and "misleading" by a High Court judge, after a challenge by Greenpeace.
The government was ordered to start again and is expected to publish the results of its second consultation next week.
But on Friday the Nuclear Consultation Working Group, made up of 17 academics and scientists including some independent government advisers, said it should have been "clear, integrated, independent and conducted over a long time-frame".
"Failure to do so has left the government vulnerable to legal challenge and may lead to hostility and mistrust of any future energy policy decision," it said.
It said key assumptions underpinning the government's approach "were designed to provide particular and limited answers". It added that key issues such as uncertainty about nuclear fuel supply, radiation risk and health effects "were not consulted on in any meaningful way".
The report concluded: "Our conclusions and recommendations to government are based on the understanding that the 2007 nuclear power consultation has failed.
"Poor consultation practice wastes people's time and can seriously undermine people's trust in government."
The group recommends that the government carry out a third consultation, addressing the issues it has raised.
Greenpeace director John Sauven said: "For such senior insiders to be so critical of the consultation process is a deeply troubling commentary on the government's approach to this issue."
"Our lawyers are looking at this report and will also examine the government's decision on new nuclear build with great interest. We won't be rushed into a decision, but nothing has been ruled out at this stage."
The government is due to publish the second consultation's findings and its own response next week.
A Department for Business, Enterprise and Regulatory Reform spokesman said the consultation, which attracted 2,700 responses, had been an "open, fair and full" process.
He said it set out the government's "preliminary view" that energy companies should be given the option of investing in new nuclear power stations and, at five months, was longer than the usual consultations.
The spokesman said a decision was needed as climate change was "accelerating" and domestic fuel supplies are running out.
"Time is pressing. Consulting indefinitely is not an option. We need to make a decision on whether we should continue to get some of our electricity from nuclear, which is a low carbon form of making energy."