Liberal Democrat peer Shirley Williams has said she is willing to be Gordon Brown's nuclear proliferation adviser "subject to conditions".
Baroness Williams said she would accept the post, on certain conditions
She told the BBC she would accept the prime minister's offer, as long as she is allowed to retain her independence.
She also said she wanted the "right to criticise government policy".
Mr Brown has pledged to "reach out beyond narrow party interest" but was rejected when he offered former Lib Dem leader Lord Ashdown a Cabinet post.
Baroness Williams, who left Labour in the early 1980s as one of the founders of the Social Democratic Party, has said at no stage was she offered a ministerial post.
Talking on BBC News 24's "Straight Talk With Andrew Neil", Lady Williams said she backed calls for the eventual abolition of nuclear weapons.
And she said there needed to be "a clear commitment by the nuclear powers that they'll move towards a substantial reduction".
Britain's Trident missile system and the Vanguard submarines which carry them need replacing by 2024.
Mr Brown angered some Labour MPs last year by signalling he wanted to replace Britain's "independent nuclear deterrent".
In March the government won MPs support to renew Trident, despite a large Labour rebellion and Lib Dem opposition. Tony Blair argued that it was "essential for our security in an uncertain world".
The Lib Dems wanted to delay a decision on replacing Trident until at least 2012.