Safety fears may halt the assembly of Britain’s nuclear warheads unless sweeping improvements are made soon, it has emerged.

The Office for Nuclear Regulation has ordered immediate safety changes to be made at the UK’s nuclear warhead assembly facility at the Atomic Weapons Establishment (AWE) Burghfield, in Berkshire, its annual report reveals. The ONR warns that, even with the changes, operations at the Berkshire site will be allowed to continue only for a limited period.

If enough progress is not made in reducing safety risks at the top-secret facility, the regulator has said, operations may need to stop completely – a move that could have serious consequences for equipping the submarine fleet with nuclear weapons.

In its report, the ONR warned that both Burghfield and its sister site, AWE Aldermaston, continued “to rely on the use of ageing production facilities in an environment of uncertainty on scope and delivery timescales for modern standard replacements”.

It said it expected AWE to continue its work on improving safety while it was subject to “an enhanced level of attention” and that it would allow “operations in the short term subject to the implementation of the most significant upgrades”.

But it warned: “We will reconsider this decision if AWE fails to deliver these key upgrades.”

The ONR recently announced it was keeping both of AWE’s Aldermaston and Burghfield plants in “special measures” – for a sixth consecutive year in the case of Aldermaston and the third for Burghfield. The watchdog has taken enforcement action five times at AWE since 2016, including over the failure to plan for dealing with 5,000 drums of nuclear waste stored at Aldermaston.

Last month the ONR announced it would prosecute AWE under the Health and Safety at Work Act following an electrical incident in June 2017 when an employee was injured. Warhead assembly work at Burghfield was halted for the first time in AWE’s history a decade ago because of safety concerns. More than a thousand safety defects were discovered during inspections, while deadlines for 300 improvements were missed.

In May the National Audit Office revealed that a project to upgrade facilities at the plant was six years late and would cost at least £1.8bn, almost 50% more than the £734m that was approved for the project in 2011.

The ONR decision comes at a crucial time for AWE, which is currently upgrading the UK’s stock of warheads. Experts suggest a shutdown of operations at Burghfield could disrupt production of warheads for the submarine fleet HMS Vanguard, currently undergoing maintenance at Devonport, Devon, and due back into service in 2019. “It is good that the regulator has stepped in, but this situation should never have happened in the first place,” said David Cullen, director of the Nuclear Information Service, a pressure group.

“It is completely unacceptable that nuclear weapons are still being built in an antiquated facility like this. The public need to be certain that if the MoD can’t deliver its upgrades to the nuclear weapons programme on time and to budget they will not just cut corners on safety instead. What we have seen so far is not encouraging.”

AWE said it had been taking steps to improve safety at Burghfield.

Its chief executive, Iain Coucher, said: “Everybody at AWE is committed to achieving high standards in safety performance. In an industry like ours, our aspirations around safety are to be leaders, nothing less.”