The cost of the government’s £38bn nuclear plant in Suffolk is subject to “significant uncertainty” and may outweigh the benefits for UK households until at least 2064, according to the government’s spending watchdog.

The National Audit Office (NAO) has warned that although the potential benefits of the Sizewell C nuclear plant are considerable, they remain uncertain. The risks, however, are “immediate, substantial and borne by the public”.

The government claims the nuclear reactor, expected to generate the equivalent of enough low-carbon electricity to power 6m homes when it begins operations in the late 2030s, could save £2bn a year from the electricity system compared with using other low-carbon technologies.

However, for households the overall savings could be outstripped by the cost of supporting its construction until almost halfway through its 60-year operational life. The project could take even longer to “break even” if there are cost overruns or delays, the NAO warned.

“Sizewell C is a project of exceptional scale, complexity and significance for taxpayers,” said Sir Geoffrey Clifton-Brown, the chair of the public accounts committee, which oversees the work of the NAO. “Experience from comparable nuclear projects in the UK and overseas highlights their vulnerability to delays and cost overruns.”

Sizewell C is being developed by French state nuclear company EDF as a successor project to the Hinkley Point C reactor in Somerset, the first nuclear plant to be built in the UK in a decade. It has invested £1.1bn to take a 12.5% stake in the project alongside the UK government, which has invested £14.2bn as the majority stakeholder.

British Gas’s parent company, Centrica, owns 15% of Sizewell C while the Canadian pension fund La Caisse and the investment fund Amber Infrastructure own 20% and 7.6%, respectively.

Nigel Cann, the chief executive of Sizewell C, said the cost on household bills were an “investment in lower long-term electricity costs” which will “deliver value to consumers and to the country for the rest of this century”.

The project is already adding value to the UK by creating thousands of jobs and boosting businesses across the country, Cann added. Sizewell C said it had so far come good on its promise to source 70% of its construction value from UK suppliers and had spent just under £5bn.

“All major infrastructure projects involve uncertainty, and the report highlights the steps we’re taking to reduce risk and control costs,” Cann said.

A government spokesperson said investing in large-scale nuclear power was the “only way to get our country off the rollercoaster of volatile global gas markets”.

Households began paying for the Sizewell C project via home energy bills at the start of the year to help fund construction. This financial framework, known as a regulated asset base model, is a marked change from the Hinkley Point deal, which will begin to earn a guaranteed stream of revenues from home energy bills only once it begins generating in the early 2030s.

Critics of the regulated asset base model, including the campaign group Stop Sizewell C, have warned that any construction delays could mean that bill payers support Sizewell without receiving power for longer than expected, while the government would be on the hook for the project’s financial risk.

Stop Sizewell C said the risks surrounding the project “could easily turn Sizewell C into a financial disaster” while the funding model meant its investors were “the only ones who can’t lose”.

The NAO has urged the government to mitigate the risk by using “close monitoring, greater transparency to parliament, and by securing value for money from the significant public and private investment”.