The former British agent known as Kevin Fulton has been arrested in London by PSNI officers.
Kevin Fulton was a British agent in the IRA
He is being brought back to Northern Ireland to be questioned about the murder of Eoin Morley and Ranger Cyril Smith in County Down.
Ranger Smith died in a bomb blast in 1990 while trying to rescue a man who had been forced to drive a car bomb into a checkpoint near Newry.
Eoin Morley, 23, was shot dead in the Derrybeg area of Newry in 1990.
The killing came amid tensions between the IRA and the Irish People's Liberation Organisation, a splinter republican paramilitary faction.