Ministers have delayed the competition to build Britain’s first mini-nuclear power plants, amid “tortuously slow” decision-making in Whitehall.

The contest to develop small modular reactors (SMRs) was whittled down to four contenders last month, with two winners originally expected to be chosen by late this year or early 2025.

That already represented a significant delay on timelines originally set out when six vendors were shortlisted a year ago.

However, the Government has now pushed back the selection of winners even further, with a decision not expected until the spring.

The two-sentence update was snuck out in Budget documents published alongside a speech by Rachel Reeves, the Chancellor, on Wednesday.

On Thursday, Great British Nuclear (GBN) confirmed the new timetable and said it would provide further updates “in due course”.

It is understood the delay is largely down to a slower-than-expected pace of decision-making in Whitehall, as well as fears that the process risks being challenged by judicial review if it is not robust enough.

One person familiar with the discussions claimed this had left GBN “shackled” by red tape. They added: “The process for making decisions is tortuously slow.”

An industry source said the change of government in July, from the Conservatives to Labour, had held up progress.

Originally, companies were promised that the shortlist of six would be reduced to four in August – but in the end this did not happen until late September.

That was after EDF dropped out and US company NuScale was eliminated from the running.

Now, four companies – Rolls-Royce, Holtec International, GE Hitachi and Westinghouse – are in negotiations with GBN for what has been billed as the final stretch of the process.