Scientists today hailed the "momentous" discovery of what appears to be the "God particle" that gives matter mass and holds the fabric of the universe together.
Teams at the Large Hadron Collider (LHC), the £2.6 billion atom smashing machine near Geneva, say they have found a new sub-atomic particle "consistent" with the Higgs boson.
The results are preliminary and more work is needed before the scientists can be sure of what "species" of particle they have captured.
But observations carried out so far show it looks and acts like the long-sought particle that has eluded them for 50 years.
Finding the Higgs is vital to the Standard Model, the theory that describes the web of particles, forces and interactions that make up the universe.
Without the Higgs boson to give matter mass and weight, there could be no Standard Model universe. If it was proved not to exist, scientists would have to tear up the theory and go back to the drawing board.
Today's announcement came at a packed seminar at the Geneva headquarters of Cern, the European Organisation for Nuclear Research, where a tense audience heard the latest progress report from the LHC.
Speaking at a London briefing with a live link to the seminar, Professor John Womersley, chief executive of the Science and Technology Research Council, said: "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."
In December last year, LHC scientists revealed they had caught the first tantalising glimpses of the particle.
But the process of proving a new piece of the universe is real is a slow and careful one, similar to getting closer to a familiar face seen from afar.
Since the initial excitement the scientists have sifted through vast quantities of data from billions of high energy collisions in an effort to reduce the chances of being wrong.
Today they confirmed that two of the LHC's giant detectors, CMS and Atlas, had delivered results that reached the definitive "five sigma" level of proof.
A sigma is a measure of how likely it is that a finding is down to chance. At five sigma, the likelihood of a statistical fluke is one in a million.
Professor Peter Higgs, the retired British physicist from Edinburgh University after whom the particle was named, was in the Geneva audience.
He dreamed up the concept of the Higgs mechanism to explain mass while walking in the Cairngorms in 1964. The unassuming Prof Higgs, who is known to shun the limelight, could now be on his way to winning a Nobel prize.
Cern director general Rolf Heuer said: "We have reached a milestone in our understanding of nature. The discovery of a particle consistent with the Higgs boson opens the way to more detailed studies, requiring larger statistics, which will pin down the new particle's properties, and is likely to shed light on other mysteries of the universe."
Professor Higgs wiped a tear from his eye as the findings were announced.
He said later: "I would like to add my congratulations to everyone involved in this achievement.
"It's really an incredible thing that it's happened in my lifetime."
The LHC, the largest scientific instrument ever built, lies in an underground tunnel with a circumference of 17 miles (27.4km) which straddles the French-Swiss border near Geneva.
Protons, the "hearts" of atoms, are fired around the ring in opposite directions at almost the speed of light. When they smash together, huge amounts of energy are converted into mass and new particles created which then decay into lighter particles.
Higgs bosons emerge from the maelstrom but only very fleetingly - for less than a trillionth of a second - before decaying. By tracing the decay patterns, the scientists were able to find the "fingerprint" of the Higgs.
One British scientist who worked on the CMS detector told how he was overcome with emotion when he heard the results from Cern.
Speaking in London, Dr Kristian Harder, from the Rutherford Appleton Laboratory at Didcot, Oxfordshire, said: "To be honest, I was listening to the transmission from Cern and I shed a tear.
"It may seem strange to someone not involved in particle physics that this is so important for us. This search has gone on since 10 years before I was born."
He said finding the particle only marked the start of a new era of discovery at the frontiers of physics.
Future research may uncover more than one Higgs boson, and find better ways of describing the universe than the Standard Model.
One theory, called "supersymmetry", called for more than five Higgs-like particles with completely different properties.
Professor Jon Butterworth, from University College London, a member of the Atlas team, said: "We don't know exactly what we've discovered but it's fundamental, new and exciting, and it looks like the Higgs boson.
"I think it's something so like the Standard Model Higgs that any differences will be incidental. We still need to see that it does the job the Higgs is there for, which is to give particles mass.
"I feel absolutely elated."
Prof Womersley said: "These results mark a significant breakthrough in our understanding of the fundamental laws that govern the Universe.
"Obviously, having found a new particle, there is still much, much more to do at the LHC - we need to confirm that this new particle is the reason some particles have tangible mass while others are insubstantial - as proposed by Peter Higgs and other scientists, who predicted that a previously unknown particle must exist for our current understanding of the Universe to work."
Science Minister David Willetts said: "This news from Cern is a breakthrough in world science. The UK has made an enormous contribution over the last 20 years supporting the search for the Higgs boson. Our researchers, universities and industry partners have been instrumental in making the Large Hadron Collider such a success.
"They deserve recognition for their contribution to this scientific milestone that will change the way we look at the universe from now on. And of course Professor Higgs of Edinburgh University has now secured his place in history."
Both the Atlas and CMS detectors found the new particle at a mass region of around 126 GeV (gigaelectrovolts) - exactly where the Higgs was expected to be.
In particle physics, energy and mass are interchangeable, and the mass of particles is described in terms of energy.
Today's seminar was a curtain-raiser for a major particle physics conference, the International Conference on High Energy Physics (ICHEP) in Melbourne, Australia.
Prof Higgs later told the BBC: "I'm rather surprised that it happened in my lifetime - I certainly had no idea it would happen in my lifetime at the beginning, more than 40 years ago, because at the beginning people had no idea about where to look for it, so it's really amazing for me to find out that it's really enough... for a discovery claim.
"I think it shows amazing dedication by the young people involved with these colossal collaborations to persist in this way, on what is a really a very difficult task. I congratulate them."
No-one can yet say whether the discovery will ever have any direct practical application.
But scientists believe the enormous multibillion-pound cost of searching for the Higgs can already be justified by spin-offs in areas as diverse as medicine, computing, electronics and manufacturing.
One of the most significant was the internet's World Wide Web, which was invented at Cern to aid communication between particle physicists across the globe.
Prof Butterworth said: "The technologies we've developed in order to find the Higgs are already benefiting us hugely. As to what we may do with the knowledge we've now gained, I'm not the person to ask."