The £20 million testing centre is expected to become the battleground in the fight between activists determined to stop its construction, and the government-backed research industry.

The university struck a defiant blow in announcing that building work had recommenced but, amid high secrecy and security at the site, refused to release details. Sources believe that the Department of Trade and Industry has contributed towards its security bill, but the university refused to comment on this.

Protesters, buoyed by their success in forcing the imminent closure of a guinea-pig farm, vowed to mobilise supporters from across the country. Police vehicles circled the half-finished laboratory yesterday. Officers formed protective cordons around builders, who covered their faces while delivering equipment to the site.

David Holmes, the university registrar, said that there had been a “great deal of security and planning” before work had resumed on the biomedical research facility. It was supposed to have opened this autumn, but the university will no longer give a deadline.

Mr Holmes said: “We had the police very closely involved. I believe it will be a very secure facility. The university remains firmly committed to the completion of this building, which will be good for animal welfare, medical research and the treatment of life-threatening conditions all over the world.” He said that the university accepted the rights of protesters to voice their opposition but that threats and intimidation were unacceptable. The university expects that 98 per cent of the animals tested will be rodents and fish, but there may also be amphibians, ferrets, rabbits and primates.

The animal rights group Speak, which has opposed laboratories in Cambridge and Oxford, said: “Oxford University is going to have a very, very strong fight on its hands. We have plans to identify the contractors and the builders and to inform them what it is they are building.”

The university said that if intimidation continued it could seek to tighten a High Court injunction that restricts protesters’ activities. It has been granted an exclusion zone, protecting university buildings and the homes of staff, students and contractors, after a campaign of criminal damage, trespass and threats. This included the posting of the home addresses and phone numbers of Oxford dons and government ministers on the internet. But intimidation has continued since the court order: a university boathouse was firebombed, and building companies have received threatening letters.

Peaceful demonstrations are permitted in a designated area on Thursdays, and activists intend to turn out in force today. The complex is intended to replace existing laboratories, bringing them under one roof. The Montpellier Group, which was contracted to build the complex, pulled out last July.

Despite a freedom of information request by The Times, the Government has refused to disclose whether it has given financial help towards the security bill. It said that revealing the data could prejudice policing at the site and has not responded to an appeal lodged three months ago. Lord Sainsbury of Turville, the Science Minister, said: “The Government is committed to tackling the menace of animal rights extremism.”

The announcement was welcomed by the Association of the British Pharmaceutical Industry, the Research Defence Society and the Association of Medical Research Charities.

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