Sir Ranulph Fiennes wants four masked men who took part in an assault that left a hunt monitor with a broken neck to be caught and brought to justice.
Two men - George and Thomas Grant - have been prosecuted for instigating the attack but they have refused to identify their accomplices.
The two attack victims were monitoring the Belvoir Hunt in Leicestershire.
Sir Ranulph and a fellow anti-fox hunt campaigner intend to raise money and offer a reward for information.
They had previously written to the attorney general asking for the suspended sentences given to the Grants to be reviewed on the grounds of undue leniency.
However, the attorney general's office said it was "not possible" to refer the sentences to the Court of Appeal for review.
Sir Ranulph said his "campaign to get justice" would continue.
"The evidence clearly showed that these two criminals called on four masked thugs to come and help them carry out their brutal attack," he said.
"How on earth can letting those responsible walk free from court be considered justice?"
The Grants were given suspended sentences after Princess Diana's sister, Lady Sarah McCorquodale, gave a character reference in court, arguing George Grant would lose his job and home if he were jailed.
Sir Ranulph and fellow campaigner Eduardo Gonçalves want people to come forward with information about the four masked men.
Mr Gonçalves said: "There are four violent criminals on the loose. If they are not caught - and jailed - they continue to pose a serious threat to the public."
What happened during the attacks?
Darryl Cunnington and his colleague Roger Swaine, who work for the League Against Cruel Sports, were attacked when they were monitoring the Belvoir Hunt in Leicestershire in March 2016.
They were discovered by Belvoir Hunt terrierman George Grant and his son Thomas Grant, who approached them on a quad bike.
George Grant then told his son: "Go and get the boys and come back."
Thomas Grant left on the quad bike and returned shortly after, accompanied by four masked men in a 4x4 vehicle who attacked Mr Cunnington and pushed him off a 14ft drop, breaking his neck.
The Grants attacked Mr Swaine themselves and stole his camera.
Who were the masked men?
The Grants admitted inflicting grievous bodily harm against Mr Cunnington and actual bodily harm against Mr Swaine, and were both given suspended sentences.
The Grants refused to answer questions when they were interviewed by police and have never identified who their masked accomplices were.
Mr Gonçalves said this lack of co-operation was a further reason why the judge should not have suspended the sentences.
Why can't the sentences be reviewed?
A petition by Sir Ranulph and Mr Gonçalves has attracted more than 85,000 supporters calling for the sentence the Grants were given to be reviewed.
However, the attorney general's office has said it does not have the power to refer the sentences to the Court of Appeal.
This is because the most serious charge the Grants were convicted of was inflicting grievous bodily harm, which does not fall under a list of offences covered by the Unduly Lenient Sentence scheme.
If the Grants had been convicted of causing grievous bodily harm with intent, which is a slightly more serious charge, then their sentences could have been referred to the Court of Appeal.
The Grants were originally charged with robbery for stealing Mr Swaine's camera, but they instead agreed to plead guilty to causing actual bodily harm and theft.
If they had been convicted of the original robbery charge then the sentences could have been referred to the Court of Appeal.