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By Joshua Nevett & Jonathan Blake
BBC News
Conservative leadership hopefuls will need the support of 100 MPs to enter the contest to be the next prime minister, under rules decided by party bosses.
Ms Truss's resignation after 45 days in office has set off a scramble to find a successor by next Friday.
Under party rules, only three MPs can run and Tory members will vote online if there is a final two.
If one candidate is picked by MPs, they will become party leader on Monday.
The contest is clearly geared towards narrowing the field as quickly as possible.
It also looks like the rules have been drawn up in the hope of MPs picking a winner without needing a final vote among party members.
Even if two remain to go head-to-head among members, there'll be an "indicative vote" among MPs so the Tory grassroots will be in no doubt about the parliamentary party's preferred leader in waiting.
Allies of former Prime Minister Boris Johnson, former Chancellor Rishi Sunak, and Commons Leader Penny Mordaunt are urging them to stand for the leadership.
Usually, Tory MPs vote to whittle down a field of leadership candidates to two, who progress to a run-off decided by the party's membership.
But there is little appetite for another long and divisive leadership contest so soon after Ms Truss won the last only six weeks ago.
Instead, Conservative Party bosses have set a higher bar of 100 nominations for Tory MPs to enter the race to replace Ms Truss.
Sir Graham Brady, the chairman of the 1922 backbench committee of Conservative MPs, said the threshold "should be achievable by any serious candidate".
Announcing the rules, Sir Graham said MPs' nominations for candidates will close at 14:00 BST on Monday.