Prime Minister Boris Johnson said dozens of Stop Sizewell C protesters, who gathered outside the nuclear site where he delivered his final policy speech before leaving office, were “wrong”.

But the executive director of the Stop Sizewell C campaign group described Mr Johnson’s promise of £700 million of taxpayers’ money to the new project as “just a gesture”, adding that his visit “may turn out to be the kiss of death for Sizewell C”.

The campaigners lined the road leading to the entrance to the Sizewell nuclear site in Suffolk as Mr Johnson was driven past in a black Range Rover on Thursday.

Protesters began to leave after learning that Mr Johnson was departing the site by helicopter, according to Alison Downes, executive director of Stop Sizewell C.

The 57-year-old, of Theberton, around four miles from Sizewell, said around 100 people turned out to show their opposition to the proposed new reactor.

Their placards read: “Boris’ last mistake”, “Nuclear disaster ahead” and “Wrong decision but it’s not too late”.

Speaking after Mr Johnson’s visit, Ms Downes said £700 million is “neither here nor there” in terms of the total cost of the Sizewell C project, which could be around £20 billion, according to reports.

“It’s not a big chunk of equity in the project; it’s too much to get the project to a final investment decision,” said Ms Downes.

“So it just seems to be a gesture to really nail the next government to a commitment to Sizewell C.

“But, actually, if I was EDF I’m not sure I would have wanted Boris Johnson to come here and give me his blessing today.

“Because I think the blessing of an outgoing Prime Minister who is known for a love of massive infrastructure projects, most of which have been consigned to the bin – I’m thinking garden bridge, the bridge to Northern Ireland, Boris island – and that’s exactly where Sizewell C belongs, so it may turn out to be the kiss of death for Sizewell C, to have him come here today.”

Mr Johnson acknowledged the protesters during his speech at Sizewell.

He said: “It’s not even as though we have some cultural aversion to nuclear power.

“Looking at those nice protesters outside, it wasn’t kind of, ‘Atomkraft? Nein, danke!’

“It wasn’t some lefty thing.

“They seem to be objecting to the disruption.

“Pure nimbyism out there.”

During a media question and answer session, he added: “I saw the protesters outside.

“I disagree with them.

“I think they’re wrong.

“I think that the disruption that they’re going to experience is going to be short and it will be worth it for the country.

“Think of those six million homes that are going to have electricity as a result of this incredible project.

“Think of the difference it will make to people in this country.”

Ms Downes said Mr Johnson “clearly knew nothing about the basis of our concerns”.

“We had banners saying, ‘We’ll all pay’, and that is the fact, for every single household in the UK is going to have to pay a nuclear tax on their bills in order to make Sizewell C financeable,” she said.

She added: “Our view of this project is it’s the wrong project.

“It’s too slow, costly, expensive and damaging and it’s in the wrong place.

“We’re in an area of outstanding natural beauty next to internationally protected RSPB Minsmere reserve and this is not the right place for a new nuclear power station.”

After making his speech, Mr Johnson toured the turbine hall of Sizewell B with executives from energy firm EDF before leaving the site.