Sir Keir Starmer has hailed the development of a small modular nuclear reactor and artificial intelligence (AI) growth zone in Wales as "probably the biggest announcement for a generation" that would bring jobs to the area "for decades to come".
The Prime Minister said more than 6,500 jobs would be created as he visited Anglesey on Thursday.
The 海角视频's first small modular reactor (SMR) nuclear power station will be built at Wylfa in North Wales by publicly owned Great British Energy -Nuclear and is backed by £2.5 billion investment from the Government.
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Ministers also announced the latest AI growth zone will be established in the region.
Sir Keir said the "absolutely amazing" data centre will be a "magnet to bring in even more jobs".
Downing Street and Energy Secretary Ed Miliband earlier defended the nuclear plant plans after criticism from US ambassador to the 海角视频 Warren Stephens, who said Washington was "extremely disappointed" by the decision.
Mr Miliband said he made "no apologies" for the 海角视频's decision to choose Britain's Rolls-Royce to design the reactor, after demands from Donald Trump's administration for a US manufacturer to be chosen.
A No 10 spokesman said the project "doesn't close the door to a larger plant elsewhere" with US involvement.
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And the Prime Minister insisted the decision to back SMR technology at Wylfa was not an "either/or".
"It is not an SMR or wider nuclear. We are doing to do both," he added.
Sir Keir said the nuclear site would create 3,000 jobs and the AI growth zone 3,500.
In a speech at a further education college, the Labour leader said: "Putting that together, 6,500 jobs – I personally think it'll be more than that because I do think these things have a magnet effect. They draw in other businesses.
"This is probably the biggest announcement for a generation, I think, here. And these are not jobs that are going to last one year or two years or three years... These are jobs that'll be there for decades to come."
Asked when the work would begin, Sir Keir said: "Some of those jobs will already be beginning and they will ramp up during the rest of this year, but then next year to come on stream for both the nuclear and the AI growth zone."
The reactors will be designed by Rolls-Royce SMR, subject to final contract.
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US ambassador Mr Stephens issued a strong rebuke of the plans, after pushing for a larger reactor at Wylfa. The US firm Westinghouse had reportedly presented plans to the 海角视频 Government to build a new gigawatt station at the site.
The Energy Secretary told Times Radio: "The reason he doesn't think it's so great is because he wanted a US company to be at this site.
"Because he thinks it's the prime site ... I totally understand that, it's completely legitimate for him to be making the case for a US company.
"We chose a British company, Rolls-Royce for this site, and I make no apologies for that.
"Our job is to stand up for the national interest."
Mr Miliband said the other site that had been considered, at Oldbury in Gloucestershire, could "accommodate various other US companies" interested in building new nuclear in the 海角视频.
A Downing Street spokesman told reporters: "It's totally legitimate for the US to say they wanted a US company there. We decided that this site should be for our own sovereign capability, for our flagship programmes, for Rolls-Royce.
"We'll be working closely with our US partners. They remain vital to Britain's nuclear future, whether that's through supply chains, technology or future sites".
Locating the small modular reactor at Wylfa "doesn't close the door to a larger plant elsewhere", the official said.
Mr Stephens explained his dissatisfaction with the decision on Wylfa was because there are "cheaper, faster and already approved options to provide clean, safe energy at this same location".
He added: "If you want to get shovels in the ground as soon as possible and take a big step in addressing energy prices and availability, there is a different path, and we look forward to decisions soon on large-scale nuclear projects."
The Government also announced Great British Energy-Nuclear (GBE-N) has been tasked with identifying potential sites for another large-scale nuclear power plant, similar to those being built at Hinkley Point in Somerset and Sizewell in Suffolk.
It will report back by autumn 2026, and has been asked by Mr Miliband to consider sites across the 海角视频, including in Scotland, officials said.
The 海角视频 and US agreed a nuclear partnership in September, potentially worth around £76 billion.
Small modular reactors (SMR) are mini nuclear power stations which are smaller and designed to be installed on site as prefabricated modules, with hopes the technology will be quicker to build than more traditional plants such as Hinkley Point C.
The Department for Energy Security and Net Zero (DESNZ) said the announcement would allow Wylfa, where previous attempts to bring back nuclear power to the site of a former reactor had failed, to become a "beacon" of a nuclear golden age, the department said.
The 海角视频's first small modular nuclear reactors are expected to generate enough electricity to power three million homes.
The move is part of plans to shift to nuclear power and other clean energy, such as renewables, which the Government hopes will bring down prices and boost the 海角视频's energy sovereignty.