Michael Portillo is expected to announce his long-awaited bid for the British Conservative Party leadership.
The shadow chancellor and front-runner is to make a statement on his intentions in Westminster today.
A supporter said: "Michael's mind is made up about what he is doing. He is pleasantly surprised and encouraged by the support he has received this week."
But the prospect of a "dream ticket", that would see him team up with former chancellor Ken Clarke, is in doubt after talks between them reportedly stalled.
Mr Clarke is in London canvassing colleagues on the best course of action. He has said it will be "a week or two at least" before he decides whether to stand.
Mr Portillo would steal a march on possible rivals by being the first to openly declare his intent to stand.
Shadow home secretary Ann Widdecombe has said she is taking soundings among MPs to see if she could attract sufficient support for a successful leadership bid.
Shadow defence secretary Iain Duncan Smith has said he has not yet decided whether to throw his hat into the ring.