Scientists believe they have captured the elusive "God particle" that gives matter mass and holds the physical fabric of the universe together.
The historic announcement came in a progress report from the Large Hadron Collider (LHC), the £2.6 billion "Big Bang" particle accelerator at the centre of the hunt for the Higgs boson.
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.
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"Discovery is the important word. That is confirmed.
"It's a momentous day for science."
Finding the Higgs plugs a gaping hole in the Standard Model, the theory that describes all the particles, forces and interactions that make up the universe.
If the particle was shown not to exist, it would have meant tearing up the Standard Model and going back to the drawing board.
In December, LHC scientists revealed they had caught a first tantalising glimpse 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 innumerable high energy collisions in an effort to reduce the chances of being wrong.
Today's news was broken at a packed seminar at the Geneva headquarters of Cern, the European Organisation for Nuclear Research.
Professor Peter Higgs, the retired British physicist from Edinburgh University after whom the particle was named, was present. He dreamed up the concept of the Higgs mechanism to explain mass while walking in the Cairngorms in 1964.
The LHC, the largest scientific instrument ever built, lies in an underground tunnel with a circumference of 17 miles that 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.
Today's seminar was a curtain raiser for a major particle physics conference, the International Conference on High Energy Physics (ICHEP) in Melbourne, Australia.
Without the Higgs boson, a Standard Model universe could not exist. Everything would behave as light does, floating freely and not combining with anything else. There would be no atoms, made from conglomerations of protons, neutrons and electrons, no ordinary matter, and no planet Earth.