Lib Dem Baroness Williams has been asked to advise Gordon Brown on nuclear proliferation, the BBC has learned.
Baroness Williams said she was thinking about the offer
Baroness Williams told the BBC she had a "friendly conversation" with Mr Brown within hours of his appointment.
She has yet to accept the advisory role, but has informed Lib Dem leader Sir Menzies Campbell.
The offer comes after the new prime minister began his premiership by saying he would "reach out beyond narrow party interest".
Baroness Williams, who left Labour in the early 1980s as one of the founders of the Social Democratic Party, was adamant that at no stage was she offered a ministerial post.
She told the BBC that she had not received a response from Sir Menzies.
Mr Brown had reportedly asked former Lib Dem leader Lord Paddy Ashdown to be Northern Ireland Secretary but was rejected.