Letter bomb attacks like the one which injured a woman at an Abingdon firm could continue, according to an Oxford-based animal rights group.

The woman was hurt when the device exploded on Thursday as she opened mail at Orchid Cellmark, based at Abingdon Business Park.

A second device was spotted before it could go off, at Culham-based LGC Forensics and a third was sent to a DNA testing firm in Birmingham, but no-one was seriously hurt. Thames Valley Police are investigating, but so far no arrests have been made.

Speak, which has led the campaign against the construction of Oxford University's £18m animal experiments laboratory, denied any involvement, and the devices have not yet been claimed by any Animal Liberation Front cells.

Speak has always pledged to keep its protests lawful and peaceful but spokesman Emma Speed warned that last's weeks bombs may not be the last.

She said: "We are not going to condone or condemn these activities but it has become increasingly difficult to protest lawfully in Oxford because of a growing number of injunctions.

"If they can not protest legally, people will seek other ways to protest, and some people will go to these lengths.

"I was arrested for taking part in a peaceful demo at a degree ceremony in Oxford in June. I was arrested for offences under the Public Order Act and bailed and I'm due to be tried in April. Quite a lot of us are now banned from Oxford, simply for taking part in legal protests."

Last month, protesters held a demonstration at the Heyford Hill junction on Oxford's Southern Bypass because many are banned from entering the city. Last May, an exclusion zone around the laboratory was extended along South Parks Road to St Cross Road and down Mansfield Road.

Protesters are no longer be allowed to use megaphones during their weekly protests outside the lab.