Dominic Cummings, the former special adviser behind the Vote Leave campaign, has urged the UK government to keep the country in the European nuclear safety and cooperation agreement (Euratom) as part of its Brexit plans.
Cummings, a close ally of Environment Secretary Michael Gove, said the Conservatives were "morons" for attempting to drag the UK out of the treatise, which it joined in 1973.
The plan, first unveiled in a White Paper in February, is because the EU and Euratom are "uniquely legally joined", according to Science Minister Jo Johnson.
But Cummings has urged Johnson's Brexit-becking brother, Boris, the Foreign Secretary, to intervene. "Boris, Gove, [Greg] Clarke, anybody sentient, tell [Prime Minister Theresa] May/DD [Brexit Secretary David Davis] TODAY this is UNACCEPTABLE BULLS**T & must be ditched," he told his Twitter followers.
Cummings, in reference to a group of pro-Leave Tory MPs, added: "Ignore whining from a small core of MPs who wd have destroyed Leave cmpgn if they'd controlled it, they have [less than]15% support in country."
The cross-party Business, Energy and Industrial Strategy Committee has also raised concerns about the UK's plan to quit Euratom.
"We share the concern of the nuclear industry that new arrangements for regulating nuclear trade and activity will take longer than two years to set up. We therefore recommend that the government seeks to delay exit from Euratom, if necessary, to be certain that new arrangements can be in place on our departure from the EU," the group of MPs said in May.
"Many of the biggest decisions in relation to energy and climate change policy are long term and not dependent on the outcome of negotiations.
"Brexit must not be allowed to distract the Government from the delivery of essential and long-term domestic policy decisions, such as the Clean Growth Plan, nor to undermine the Government's commitment to meeting emissions reduction targets, which are enshrined in domestic legislation.
"We recommend that the Government provides a clear and long-term vision for the UK energy sector to support investor confidence and that this should underpin its negotiating objectives."
Cummings' latest intervention comes at a key time for May, with her one-year anniversary as prime minister on Tuesday 11 July and Great Repeal Bill being tabled in parliament on Thursday 13 July.
The draft piece of legislation is designed to transfer EU law into the UK statute book on the day of Brexit, expected at some point in 2019.
With the Conservative majority in the House of Commons wiped out at the general election, May has urged opposition parties to "contribute" to the government's Brexit plans. But the Great Repeal Bill will give the MPs and peers the opportunity to scupper and even derail the prime minister's plans.
"The PM is promising to take big decisions in the long-term interests of Britain," said Liberal Democrat Brexit spokesman Tom Brake MP. "Her first such decision must be to negotiate to keep the UK in the Single Market and the Customs Union. This is in the long term interests of British prosperity, jobs and families.
"A bad deal or no deal at all, will do permanent damage to our economy which is already affected by Brexit blues."