Police have warned the university to tighten up its vetting checks on new staff, and college deans, who believe that attacks are inevitable, have ordered students to sign in night-time guests with gate porters.

In a letter seen by The Times, the Dean of Pembroke College, Adrian Gregory, acknowledges that students may regard the tighter restrictions as an invasion of privacy. But he says that he fears that it is necessary because activists will conduct reconnaissance trips to identify the easiest targets among Oxford’s colleges.

Builders wearing masks to hide their identities restarted work on the university’s £18 million biomedical research laboratory in December, 16 months after intimidation and threats to contractors and shareholders suspended its construction.

The planned facility has become the main battleground in the fight between activists and those involved in animal research — the Animal Liberation Front (ALF) recently announced that anyone associated with Oxford University, including students, was a legitimate target for attack.

Security experts have reportedly been recruited to protect key workers at the site of the new laboratory.

A college boathouse was burnt down in an arson attack last July, despite the granting of a High Court order creating a no-protest zone around the university and homes of staff and students.

The ALF has this month falsely accused executives of GlaxoSmithKline, Britain’s largest pharmaceutical company, of being convicted rapists. Activists sent letters to the men’s neighbours, purporting to be from a police officer.

Thames Valley Police has written to all Oxford colleges, giving details of its security advisory programme to reduce potential risk. It recommends stricter vetting procedures and scrupulous reference checks before employing staff, particularly temporary workers.

A police spokesman said: “We are doing all we can to assist colleges in this and are there to provide advice if needed. We’ve offered our services and a number of colleges have taken us up on it.

“Colleges should be aware of what staff they’re taking on. We don’t want keys being copied, which would be a nightmare situation.”

Dr Gregory told its student committee: “The ALF declaration of war is a real cause for concern.”

He said it was likely that “colleges are going to be targeted, probably for vandalism, but we can’t rule out worse”.

“Logically any attack is going to happen at night. In these circumstances, I don’t think we have any choice but to crack down on gate security. I’ve told the porters that we will need to seriously police the practice of students holding the door open for strangers at night.

“I’m authorising the porters to send me names of anyone caught doing this. I know that this can seem a bit officious on the part of the porters, but the concerns are very real.”

“I realise that holding a door open is frequently a reflex of politeness but we do have to be careful. In essence students need to think of our front gate as their front door at home.”

Dr Gregory said that the college would be more vulnerable if it appeared to be a soft target.

“My guess is that activists will soon be mounting ‘reconnaissance’ in order to see which colleges they can hit,” he said.

“It is also imperative that legitimate guests after midnight are signed in, and out if they leave.

“We have no interest in what students do in their private life and we simply need to know that anyone who is in the college at night has a student taking responsibility for them.”

Many colleges have a signing-in system, but it is not always rigorously enforced.

• Six animal rights extremists are on trial in America for an alleged terrorist-style campaign against Britain’s biggest testing centre (Nicola Woolcock writes). They face up to 13 years in jail for trying to shut down Huntingdon Life Sciences (HLS), which is based in Cambridgeshire but has a laboratory in New Jersey. The five men and a woman face charges of animal enterprise terrorism, conspiracy and stalking.

  Prosecutors say that their website incited violence and encouraged attacks against HLS and its employees. Jury selection began this week at the trial in Trenton, New Jersey, while about 70 animal rights protesters demonstrated outside.

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