Candidates to replace Liz Truss as Tory leader will need at least 100 nominations from Conservative MPs, 1922 Committee chair Sir Graham Brady has said.
This will rule out a number of candidates from running, and means the maximum number of people able to stand is three.
During the last leadership election, Rishi Sunak won 137 nominations, Liz Truss 113 and Penny Mordaunt 105.
“We fixed a high threshold but a threshold that should be achievable by any serious candidate who has a prospect of going through,” Sir Graham said.
Nominations are open from now and will close at 2pm on Monday.
The final two candidates will take part in a hustings event organised with news broadcasters.
Conservative Party chairman Jake Berry said there would be an online vote for members if two candidates made it through the parliamentary stages.
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Sir Graham has already said that the new prime minister will be chosen by Friday 28 October, with Ms Truss to stay on as PM until then.
She announced her resignation earlier on Thursday after she met Sir Graham and agreed for a leadership election “to be completed within the next week”.
It means her successor will be in place before the crucial fiscal statement on 31 October.
After 44 days in the top job, Ms Truss will be the shortest-serving prime minister in modern British political history.
Her downfall was set in motion by her disastrous mini-budget, which sparked turmoil in the financial markets and forced her to U-turn on the tax-slashing agenda that brought her into office.
In her resignation statement outside Downing Street, Ms Truss said she recognised she could not “deliver the mandate on which I was elected by the Conservative Party”.
Speculation is mounting about who could replace Ms Truss, with many Conservative MPs calling for Boris Johnson to return.
But any comeback from the ex-PM is likely to be met with a backlash, with other Tories describing such a move as a “fantasy” and “too soon”.
