Scientists at Europe's CERN research centre have found a new subatomic particle that could be the Higgs boson.
"We have indeed discovered a particle consistent with the Higgs boson," said John Womersley, chief executive of Britain's Science & Technology Facilities Council, at an event in London.
"These results mark a significant breakthrough in our understanding of the fundamental laws that govern the universe."
Joe Incandela, spokesman for 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."
Addressing the scientists assembled in the CERN auditorium, Heuer asked: "Would you agree?" They burst into applause.
Peter Higgs, the 83-year-old British physicist who proposed the existence of the Higgs boson in the 1960s, was at CERN to welcome the news.
He told the symposium: "It is an incredible thing that it has happened in my lifetime."
The Higgs theory explains how particles clumped together to form stars, planets and life itself.
Without the Higgs particle, the particles that make up the universe would have remained like a soup, the theory goes.
It is the last undiscovered piece of the Standard Model that describes the fundamental make-up of the universe.
The model is for physicists what the theory of evolution is for biologists.
What scientists do not yet know from the latest findings is whether the particle they have discovered is the Higgs boson as described by the Standard Model.
It could also be a variant of the Higgs idea or an entirely new subatomic particle that could force a rethink on the fundamental structure of matter.
The last two possibilities are, in scientific terms, the most exciting.
Packed audiences of particle physicists, journalists, students and even politicians filled conference rooms in Geneva and London to hear the announcement.
Despite the excitement, physicists cautioned that there was still much to learn.
"We still much we don't know about particles - this is only the beginning of a new journey. We have closed one chapter and opened another," Peter Knight of Britain's Institute of Physics said.
Oliver Buchmueller, a senior physicist on one of the research teams, said: "If I were a betting man, I would bet that it is the Higgs.
"But we can't yet say that definitely yet. It is very much a smoking duck that walks and quacks like the Higgs.
"But we now have to open it up and look inside before we can say that it is indeed the Higgs."
Higgs called it a great achievement for the Large Hadron Collider, the 27-km long particle accelerator built in a tunnel underneath the French-Swiss border where experiments to search for the Higgs boson have taken place.
In a statement, he added: "I never expected this to happen in my lifetime and shall be asking my family to put some champagne in the fridge."