Construction of two key access roads for Britain’s Sizewell C nuclear power station has been delayed, and the main contractor replaced, in a setback at one of the UK’s biggest infrastructure projects. The delay threatens to prolong disruption around the Suffolk construction site and comes as Sizewell C seeks to demonstrate that it can avoid the delays and cost overruns that have plagued its sister project, Hinkley Point C in Somerset. The main contractor for the access roads, construction group Galliford Try, has been replaced after failing to agree with Sizewell C on the cost of the work.

Kier and local firm Breheny have now stepped in to replace Galliford, splitting the road-building package between them. One person familiar with the situation said there had been heavy cost overruns because of several design changes. “They asked for changes that added £30mn,” they said, referring to Sizewell C. The change of contractor was likely to delay completion of the roads by between six months and one year, the person added.

Local residents said the delay would extend disruption in villages already experiencing increased construction traffic. “The link road and bypass were mitigation to reduce the significant impacts of a decade of HGVs and worker buses travelling through multiple small villages,” said Alison Downes of campaign group Stop Sizewell C. She said there had already been an “awful lot of changes” at the project despite Sizewell C’s pledge to apply lessons from Hinkley Point C, which uses the same EPR reactor technology. “We have always said you can replicate the above-ground design but you cannot replicate the site and location,” Downes said. The nuclear project, owned by the UK government and a consortium of investors, including Canadian pension fund La Caisse, Centrica and EDF, is scheduled to start generating power in the mid to late 2030s, after a 10-12 year construction programme.

The project has said it will minimise delays by replicating much of the design of Hinkley Point C. Sizewell C said in presentations last year that it expected construction of both roads to be completed in 2026, with the Sizewell Link Road due to open in June 2027. Sizewell C’s website states that the road will be completed next year.

Nigel Cann, Sizewell C chief executive, acknowledged in evidence to MPs in June that work on the roads was running behind schedule. “We are probably a little behind in setting the link road and the two-village bypass,” Cann told the House of Commons Public Accounts Committee, adding that the project had made a “conscious decision” to secure the right price rather than “rush in with a low level of design”. Galliford Try said: “Our highways business has been unable to reach a mutual agreement with Sizewell C to construct the transport infrastructure that the project requires, following an intensive 20-month period of Early Contractor Involvement.” Sizewell C said it had not agreed commercial terms with Galliford Try, but that Kier and Breheny were “already working on our project and exceptionally well placed to deliver these crucial programmes”.