The government has warned some prominent American activists that they may be banned from entering the UK.
A major animal rights conference is organised for September
The Home Office has sent a number of warnings to campaigners that they are examining statements made by them.
US surgeon Dr Jerry Vlasak and Dr Steven Best are due to address a conference in September organised by Stop Huntingdon Animal Cruelty (SHAC).
SHAC campaigns against practices at the Cambridgeshire research centre Huntingdon Life Sciences.
The Observer newspaper had reported Mr Vlasak saying: "I think for five lives, 10 lives, 15 human lives, we could save a million, two million, 10 million non-human lives."
But Dr Vlasak furiously denied issuing a call for such action. He
said he spent his entire professional life saving lives and had not encouraged
Dr Vlasak said: "I was outraged by the article stating that I, a medical doctor who spends my entire life saving lives, was calling for or encouraging the assassination or
killing of any being, human or non human."
Mr Blunkett had asked officials to check the authenticity of the Observer's reports so that he could decide if Dr Vlasak - a top adviser to animal rights groups - should be allowed to visit Britain.
He told the BBC's Breakfast With Frost: "I am asking that we examine the authenticity of the claims in those quotes, the attitude of this individual and whether his presence would be non-conducive to the public good."