A new rule was introduced after police chiefs lost one of their undercover officers and stayed with the group he was spying on

There is an intriguing story about an undercover officer who "went native" and refused to go back to his police handlers. Can you help us piece together the tale of this one-time police spy ?

It seems that the officer was sent to infiltrate a political group and found that he very much enjoyed being with the group - so much so that when his police bosses told him to end his undercover mission with the group, he refused point-blank.

It appears that he then quit the police and stayed with the group. We are unsure when this happened but it seems to have been in the 1970s or 1980s. There are indications that the group he spied on was left-wing.

We wonder if he has ever revealed his true identity to the circle of activists and friends he stayed with or stayed silent. If you are that person, we would of course very much like to hear from you. We think that you would have an interesting story to tell.

The officer was a member of the Metropolitan police's Special Demonstration Squad which was set up in 1968 to spy on political groups. It may be that he was the first undercover officer in that squad to have gone over to the other side in this way.

It appears that the case prompted a change in the recruitment practices of the SDS. The new rule was that from then on, members of the squad had to be married.

It is unclear when this rule was created, but here Pete Black, a member of the SDS between 1993 and 1997, explains why it was brought in :"They introduced that rule after one officer refused to come out of the field. It turned out he just enjoyed being with his contacts so much that he was willing to give up his police salary and everything that went along with it in order to stay with them.

"Now you have to be married on the basis that, if you have something in the real world to come back to, you are less likely to want to remain in role. That's the theory."

Black gave this explanation when he told his story in the Observer in 2010.

As ever, any information gratefully received.

Email paul.lewis@guardian.co.uk or rob.evans@guardian.co.uk