A former IRA man, Mr Sean O'Callaghan, is to provide some advice to the Ulster Unionist leader at Westminster.
However, he would not be working in a formal capacity for Mr Trimble and there were no fees involved, a UUP official said last night.
Mr Trimble has known Mr O'Callaghan for at least five years and is to provide him with a permanent Westminster pass which will allow him unaccompanied access to many areas of the palace.
Mr O'Callaghan has worked at Westminster where, according to a source, he has helped "decode republican thinking" for backbench MPs.
It was stressed last night that the arrangement was a personal one between Mr O'Callaghan, who has also been an MI5 agent, and Mr Trimble.
Mr O'Callaghan, who has been convicted of two murders, said yesterday: "It's very much an individual decision. David has put in an application for a House of Commons pass for me. I won't be getting paid by David Trimble and I won't be sitting in his office regularly. I spend a lot of time in Westminster anyway."