Scientists at the European Council for Nuclear Research (Cern) have discovered a subatomic particle which could be the Higgs Boson, the elusive "God particle" that gives mass to all other particles in the universe.
The search for the Higgs Boson has been going on for decades at a cost of billions of pounds. Scientists have now announced they have seen the best evidence yet that it exists.
Professor John Womersley, chief executive of the Science and Technology Facilities Council, told reporters at a briefing in London: "They have discovered a particle consistent with the Higgs Boson.
"Discovery is the important word. That is confirmed.
"It's a momentous day for science."
Joe Incandela, from CMS, one of the two teams hunting for the Higgs particle, told an audience at Cern, near Geneva: "This is a preliminary result, but we think it's very strong and very solid."
Cern's director general Rolph Heuer said: "As a layman, I would say I think we have it."
Among the scientists in attendance was Professor Peter Higgs, who first proposed the idea of the particle's existence in 1964.
Professor Higgs, who could be seen holding back tears during the announcement, said: "I never expected this to happen in my lifetime and shall be asking my family to put some champagne in the fridge."
Incandela could not confirm the particle discovered by the Large Hadron Collider particle accelerator, is indeed the Higgs Boson, though he said the data has reached the level of certainty needed for a discovery.
Fabiola Gianotti, leader of the second team of some 3,000 scientists, known as Atlas, said it also has observed some "beautiful" events in the Large Hadron Collider.
Both Atlas and CMS independently discovered new particle with a mass of around 125-126 GeV (gigaelectronvolts).
"The results are preliminary but the five-sigma signal at around 125 GeV we're seeing is dramatic. This is indeed a new particle," said Incandela.
Heuer expalined the need for verification of the particle: "It's a bit like spotting a familiar face from far," he said. "Sometimes you need closer inspection to find out whether it's really your best friend, or your best friend's twin."
Crucial details, including a video, of the discovery were accidentally leaked on to Cern's website just hours before the announcement was made.
Incandela said on the video: "We have observed a new particle. We have quite strong evidence that there's something there.
"Its properties are still going to take us a little bit of time. We can see that it decays into two photons, for example, which tells us it's a boson, it's a particle with integer spin, and we know its mass is roughly 100 times the mass of the proton. And this is very significant."
According to CERN: "The next step will be to determine the precise nature of the particle and its significance for our understanding of the universe.
Scientists at the LHC first revealed they had "glimpsed" the Higgs Boson during an announcement in December.
The phrase 'God particle' was coined by Nobel Prize-winning physicist Leon Lederman. It is not used by physicists, but rather as an easier way of explaining how the subatomic universe works and got started.