An undercover police officer has been accused of encouraging and helping an animal rights campaigner to commit illegal acts which led to his being jailed for four years, according to legal documents.

The campaigner, Geoff Sheppard, has lodged an appeal to overturn his convictions for possessing a shotgun and components for an incendiary device, alleging, in effect, that he was a victim of an agent provocateur.

Sheppard said the undercover officer, whose covert role is revealed by the Guardian this Thursday, actively encouraged him to buy the shotgun and offered him money to purchase it. He claims that as part of a “determined, cynical, and targeted effort” against him, the undercover spy asked him for instructions on making an incendiary device, and tested it.

The Guardian has established that the undercover officer at the centre of the new allegations worked for a controversial covert unit that infiltrated hundreds of political groups for 40 years.

The officer, a member of the Metropolitan police’s special demonstration squad, used the fake name of Matt Rayner to pose as an animal rights campaigner for five years.

In the last three years 50 campaigners have had their criminal convictions quashed because key evidence gathered by undercover officers had been concealed from their trials.

The home secretary, Theresa May, has commissioned a lawyer, Mark Ellison QC, to examine other convictions of political campaigners to see if they should also be overturned.

She has also ordered a wider ranging public inquiry into the conduct of undercover officers following a succession of revelations, ranging from spying on the family of Stephen Lawrence to forming sexual relationships with women placed under surveillance.

The allegations made by Sheppard are contained in a legal application lodged at the appeal court and focus on Rayner, the latest undercover officer to be publicly identified.

Like the other undercover officers Rayner developed a fake identity, bolstered by false documents; he pretended to be an animal rights campaigner between 1991 and 1996. Among the campaigns he infiltrated was a London group campaigning to stop the high street chemists Boots from selling products that had been tested on animals.

Met policeman ‘Matt Rayner’ developed a fake identity and pretended to be an animal rights activist. Guardian Photograph: Guardian

Sheppard, who was one of the activists he was spying on, had come out of jail in 1990 after being convicted of setting fire to three branches of Debenhams in a campaign against the sale of fur.

In the legal application drawn up by his lawyers Sheppard said he had returned to animal rights campaigning “but did so initially on the basis of entirely lawful protest including leafleting and demonstrating at stores that were connected with animal experimentation”.

In May 1995 police raided his home in London and found a shotgun, cartridges and components for making an incendiary device. At his trial in September 1995, prosecutors said he had been planning to produce an incendiary device to further an Animal Liberation Front campaign against Boots.

Sheppard alleges that Rayner targeted him and “deliberately encouraged him to take more serious direct action against Boots” and to commit crimes he had been initially unwilling to carry out.

Sheppard also alleges that Rayner took part in direct action protests and encouraged activists by driving them in a van to and from demonstrations, such as for an occasion to sabotage a grouse shoot.

His lawyers, from the civil liberties firm Bindmans, claim that he was convicted unfairly because prosecutors withheld from his trial Rayner’s involvement and misled him into pleading guilty.

It is the second conviction that Sheppard is seeking to overturn on grounds that the involvement of an undercover officer was not disclosed at his trial.

In this second case, Bob Lambert, another undercover officer working for the SDS, secured vital intelligence in 1987 that led to the convictions of Sheppard and another activist, Andrew Clarke, for setting fire to Debenhams.

By the 1990s Lambert, who fathered a child with an animal rights activist before abandoning the pair, had been promoted to run the operations of the SDS, and he supervised Rayner.

Rayner ended his deployment in 1996, telling activists he was going to work for a wine company in France and then Argentina, before disappearing. In reality, he returned to Scotland Yard and was reassigned to other duties within special branch.

The Met said it was not “prepared to confirm nor deny” whether Rayner worked as an undercover officer. It added that the police’s internal inquiry into the SDS, known as Operation Herne, had attempted to talk to Sheppard and that it would welcome an opportunity to speak to him about his claims about Rayner.