Nobel laureates and other leading scientists have made a last-minute bid to sway government opinion in favour of building the Diamond synchrotron at Daresbury in Cheshire.
An array of scientific experts spoke out at a meeting organised to help see off the challenge from an alternative site at the Rutherford Appleton Laboratory (RAL) near Oxford.
The chemist Sir Harry Kroto and pioneering molecular biologist Max Perutz both lent their support to the Cheshire site where the United Kingdom's existing X-ray source has been established for the past 20 years.
Their arguments were voiced before an audience which included Michael Morgan, chief executive of the Wellcome Trust Genome Campus.
The Wellcome Trust, which is putting up Pounds 110 million to fund the Pounds 200 million new facility jointly with the Office of Science and Technology and the French government, would prefer the southern site.
A final decision is imminent. Stephen Byers, the secretary of state for industry, is awaiting two reports on the situation before announcing where the Pounds 175 million national X-ray source should be placed.
Dr Perutz, former director of the Medical Research Council's Laboratory of Molecular Biology in Cambridge, was unable to address Friday's meeting in person.
In a statement, he argued that British scientists would very soon need access to a powerful, modern synchrotron such as Diamond to match the American effort to take full advantage of the deciphering of the human genome.
Sir Harry said that in addition to the scientific merit of siting Diamond by its predecessor, the laboratory acted as a regional centre of excellence for science.
"It is important that such establishments should be distributed throughout the country. Young scientists can then more easily find role models and decide to pursue the careers in science and technology which will be so important for the nation's future," he said.
The trade unions at Daresbury fear that if the new facility is sited at RAL, their laboratory would close and 530 jobs could be at risk.
Mr Byers has delayed the announcement from the autumn after calling for two reports on the siting of Diamond, the first to gauge the views of synchrotron users and the second an engineering survey on the two sites.
A DTI spokesman said the two reports were expected by the end of this week and that the decision would be made within a fortnight.