Boris Johnson has promised £700million for Sizewell C in his final major policy speech as prime minister and said he was “absolutely confident it will get over the line” in the next few weeks.

In his final policy speech before leaving office, Mr Johnson hit out at successive administrations’ “paralysis” over nuclear power and pledged the funding as part of a bid to move the Suffolk scheme forwards.

The new reactor at Sizewell is expected to be built by energy firm EDF though talks over funding continue.

Mr Johnson leaves office on Tuesday but said he is confident the deal will get “over the line” in the coming weeks.

“We need to pull our national finger out and get on with Sizewell C,” he said.

“That’s why we’re putting £700million into the deal, just part of the £1.7billion of Government funding available for developing a large-scale nuclear project to final investment stage – FID (final investment decision) – in this Parliament.

“In the course of the next few weeks, I am absolutely confident that it will get over the line.”

Mr Johnson said it would be “madness” not to go ahead with the project, which would “fix the energy needs, not just of this generation but of the next”.

Mr Johnson will hand power to either Liz Truss or Rishi Sunak following the Tory leadership contest.

He said: “I say to you, with the prophetic candour and clarity of one who is about to hand over the torch of office, I say go nuclear and go large and go with Sizewell C.”

Downing Street was unable to give further details about the funding announcement, with negotiations over the final arrangements for the project continuing.

The total cost of the Sizewell C project could be around £20billion, according to reports.

It is not expected to begin generating electricity until the 2030s; the similar reactor at Hinkley Point C in Somerset began construction in 2016 and will not be online until 2027, although this is partly due to the impact of the pandemic.

He continued: “This project will create tens of thousands of jobs, it will also power six million homes – that is roughly a fifth of all the homes in the UK – so it’ll help to fix the energy needs, not just of this generation but of the next.

“A baby born this year will be getting energy from Sizewell C long after she retires and this new reactor is just a part of our Great British nuclear campaign.”

Campaign group Stop Sizewell C condemned the prime minister's visit to the Suffolk coast just days before he steps down, saying: "£700million is not enough to take a big chunk out of the equity needed for Sizewell C but it’s too much for development funds so it seems to be an extravagant gesture to nail his successor to this white elephant.

"When every penny matters, it's totally wrong to shackle the next prime minister and billions in taxpayers' money to this damaging project, whose ballooning cost, lengthy construction, failure-prone technology and long-term water supply are so uncertain.

“A truly visionary legacy for Boris Johnson would be to ditch Sizewell C, which is most definitely not green, and focus on cheaper, quicker renewables and insulation, which are.”

However, union bosses said the announcement did not go far enough.

Sue Ferns, senior deputy general secretary of Prospect union, said: “This is one more step on the way to approving a project vital to achieving our net zero goals and future energy security. But it falls short of the full Government Investment Decision we were hoping for which is required to secure the rest of the necessary funding from investors.

“If this painfully slow progress is replicated across all the nuclear projects promised in the British Energy Security Strategy, then we have no chance of achieving its ambitious aims.

“The incoming prime minister must prioritise achieving resolution, making absolutely clear Britain’s commitment to new nuclear and to this critical project.”

Jeff Knott, RSPB operation director, said: "We can't understand why a Prime Minister that claims to be tackling the nature crisis wouldn't spend some of his trip visiting RSPB Minsmere to better understand the effects the development will have on this internationally recognised and critical site for nature.

"Due to consent being granted against the recommendations of the Examining Authority and advice of government advisers, the RSPB, after years of engaging with the application feels it has no choice but to apply for leave to Judicial Review.

“For an outgoing PM concerned with his political legacy, any decision to plough on with Sizewell C regardless of all the concerns will only call in to question any environmental legitimacy he may claim.”

An EDF spokeswoman said: "The UK Government has announced its support for Sizewell C. In line with the Government’s decision we continue to work to deliver the Sizewell C project which is key to Britain’s energy security and climate change objectives.”