Attacks on an executive's home and a university show the increasing violence of animal rights extremists, a scientists' group says.
Firms linked to Huntingdon Life Sciences have been targeted
Activists have admitted firebombing the home of a Glaxosmithkline executive and an Oxford University building.
Leapfrog Day Nurseries also said it had been threatened over a childcare scheme it ran for Huntingdon Life Sciences.
Tougher laws had prompted a "die-hard" minority to adopt more violent tactics, the Research Defence Society warned.
Police are investigating the firebombing incidents on 7 and 23 September. No one was hurt.
Descriptions of both attacks were posted on the Animal Liberation Front (ALF) website Backbite.
In the first, a group calling itself the Oxford Arson Squad claimed to have planted a number of "large incendiary devices" in an empty house belonging to the university in protest at its plans to build a lab for tests on animals.
A message addressed to Oxford University read: "You cannot build the South Parks Lab without incurring massive losses."
It went on: "If we have to destroy every bit of property you own we will, in order to stop you inflicting your profit driven cruelties on defenceless creatures."
Oxford University said the attack at Corpus Christi College caused minor damage.
A spokesman said the university was "appalled" by the "intimidating nature" of the online message but remained committed to building a new biomedical research facility..
In the second posting, a group calling themselves Brigade G of the Animal Liberation Front claims to have detonated a device at the home of Glaxosmithkline executive Paul Blackburn in protest at his firm's links to Cambridgeshire-based Huntingdon Life Sciences (HLS) - the focus of an intense anti-vivisection campaign.
In a message to Glaxosmithkline, it says: "We have identified and tracked down many of your senior executives and also junior staff, as well as those from other HLS customers.
"Drop HLS or you will face the consequences. For all the animals inside HLS, we will be back."
It is understood Mr Blackburn was away when the device was left in his porch but his wife and daughter were in the house. It caused minor damage.
Leapfrog Day Nurseries said its directors received a threatening letter over a childcare voucher scheme it ran for Huntingdon Life Sciences.
The company said that while threats were unacceptable the safety of staff and children in their care were paramount so had decided to withdraw the scheme.
A spokesman for the police National Extremism Tactical Coordination Unit (Nectu) said there was an "ongoing level of animal rights activity around the country".
Dr Simon Festing, of the RDS, said the latest attacks marked a "change of style" on the part of the activists.
He told the BBC News website: "The extremists are desperate to achieve their aims yet at the same time are under much greater pressure from the police and from new legislation."
This meant tactics they had been developing were no longer effective, so a small band of die-hard extremists had switched to a more extreme approach, he added.
He said: "We're at a critical time now because the government has always called it 'extremism' but it's moving closer to what might be described as 'terrorism' - the old tactics of arson and things like that."