Animal rights activists have so far got away with committing about 20 crimes including arson, blackmail and vandalism since work restarted on Oxford University's research laboratory.

As yet, no-one has been charged with any of the offences committed since work began again on the laboratory in South Parks Road at the end of 2005.

The information was obtained by the Oxford Mail, but details of the offences have not been released.

However, some which the Animal Liberation Front has claimed responsibility for include vandalising locks, an arson attack on lorries in Witney and sending threatening letters to builders involved in the construction of the lab. Police said the cases had not been closed.

A spokesman said: "The recent conviction of Donald Currie, an animal rights extremist who left explosive/incendiary devices at two addresses within Thames Valley demonstrates very clearly that a breakthrough in such investigations can come at any time - some six months down the line in this instance.

"It is disappointing that these offences have been committed in the first place and offenders should be warned that we will fully investigate any incident reported to us. The detectives assigned to these cases are doing everything possible to bring those responsible to justice.

"Anyone committing an offence such as arson is putting others at risk, no matter what their motive. Animal rights activists are given plenty of opportunity to make their opinions heard via legal and peaceful protest or lobbying, and criminal activity will not be tolerated."

Despite the number of unsolved cases, Oxford neurosurgeon Tipo Aziz said scientists who participate in animal testing feel safer now than they did at the end of 2005 when contractors returned to the the lab.

He said: "I would expect people to have been charged for carrying out these offences, but I don't know the exact circumstances of each case. The high number of offences reflects the fact that there has been increased public support for testing and that they have no regard for the citizens of this country.

"I think the police have been very good overall. Scientists certainly feel more secure and any employees feel more open to talk about their research, whereas a year ago the attitude was 'no comment'."

Pro animal-testing group Pro-Test spokesman Iain Simpson said he felt that policing of animal rights protesters had become much more effective than ever over the last year.

He said: "At a national level there have been far fewer incidents this year than any other. The number of things that have happened at people's homes were running at a rate of about 10 a month, but they have dropped considerably.

"If you have got someone wearing a balaclava doing silly stuff in the middle of the night it is difficult to get arrests, but it is wrong to look just at Oxfordshire. There are people on a national level who have been charged in 2006."

ALF spokesman Robin Webb added: "It is not surprising none of these incidents have been solved. The ALF has no structure. It is just individuals or groups of individuals acting under a series of policies that have been in place for 30 years. Therefore there is no chain of command for the police to gain information on."