A watchdog warned of serious risks over whether the Brexit deadline could be met for putting in place a new nuclear monitoring regime, leaked documents have revealed.

An assessment prepared earlier this year set out five "high-level risks" in progress on setting up new systems to replace work carried out by Euratom, which regulates transportation and disposal of nuclear materials.

Funding, recruitment, training, equipment and a new IT system were all given a red rating on a traffic light ranking in the Office for Nuclear Regulation (ONR) analysis seen by Sky News.

Anti-Brexit campaigners said the report showed the Government's exit strategy was "frankly dangerous" and unions warned of "incredibly serious" consequences if the deadline is not met.

The new regime will not deal with safety issues, which are already carried out domestically, but covers the management of nuclear stocks.

Since the document was drawn up, good progress has been made on recruitment and enough warranted inspectors are expected to be in post by the time Britain quits the European Union in March 2019, according to the ONR.

The regulator said it expects the new IT system to be up and running by the end this year.

Sue Ferns, senior deputy general secretary of Prospect, which represents engineers and scientists, said the civil service was "racing against the clock".

"It is hard to think of anything more important than making sure the UK has a fully operational nuclear safeguards regime after Brexit.

"The consequences if we get it wrong are potentially incredibly serious," she said.

"But the reality is that the totally unnecessary decision to pull the UK out of a nuclear regime that has served us well for years has left civil servants racing against the clock to create a whole new safeguards regime from scratch."

Eloise Todd, chief executive of the Best for Britain group, which is campaigning against Brexit, said: "This looks like Brexit is about to go nuclear.

"The Government can say we will be ready by day one but the massive red warning lights in these leaked reports do not lie.

"This is frankly dangerous and the Government and the regulator need to work urgently on this.

"It seems post-Brexit Britain might not keep the lights on."

Doug Parr, chief scientist at Greenpeace UK, said: "The Government is planning to massively expand our stockpile of dangerous radioactive material with new reactors at Hinkley, Wylfa and four other sites, without a long-term storage plan.

"Now we learn their strategy for replacing the monitoring and management of that stockpile after Brexit is inadequate in every single respect.

"They don't appear to be taking the very significant dangers of radioactive materials in the least bit seriously."

In April the Department for Business, Energy and Industrial Strategy (BEIS) announced it was putting an extra £4,626,000 from the Government's contingencies fund into work setting up the new regime.

An ONR spokesman said: "The Office for Nuclear Regulation is working with the UK Government to help ensure a smooth transition of nuclear safeguards arrangements to the UK from Euratom, through the implementation of a UK state system of accountancy for and control of nuclear material which meets Government expectations and international obligations."

A BEIS spokesman said: "The Government has made significant progress in preparing to leave Euratom to ensure safeguards are in place from day one.

"The Nuclear Safeguards Bill is making good progress through Parliament and we continue to work closely with the Office for Nuclear Regulation to ensure we continue to have a robust regime in place."