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By Kate Whannel
Political reporter, BBC News
Liz Truss has announced she will resign, which means there will now be another leadership election to decide who becomes the next Conservative leader and prime minister.
The contest to replace her is expected to be completed by the end of next week.
No-one has yet confirmed that they will stand, but here are some of the potential candidates.
Rishi Sunak
Rishi Sunak ran to replace Boris Johnson as leader earlier this summer and made it to the final two along with Ms Truss, having won the most support from Conservative MPs.
During the campaign he warned that his rival's tax plans would damage the economy, but his message failed to appeal to party members and he lost by 21,000 votes.
Mr Sunak only became an MP in 2015, for the North Yorkshire constituency of Richmond. Few outside Westminster had heard of him, but he was chancellor of the exchequer by February 2020.
He quickly had to grapple with the coronavirus pandemic, spending huge amounts of money trying to keep the economy afloat during lockdown.
This didn't come easily to a man who saw himself as a low tax and spend Conservative on the Thatcherite wing of the party but it did boost his popularity.
However, his reputation was dented following a controversy over his wife's tax affairs, and not long after that he received a fine for breaching lockdown rules.
Boris Johnson
With just a week to choose a new leader, many of the contenders are likely to be familiar faces, and none more so than the man who was prime minister just weeks ago.
Boris Johnson was forced to announce his resignation in July, after a mass revolt by ministers and MPs. It followed months of rows over Downing Street lockdown parties and other controversies.
The MP for Uxbridge faces an investigation by the Privileges Committee into whether he obstructed the Commons by telling MPs that lockdown rules had been followed at No 10. He and others were subsequently fined for Covid breaches.
However, he still has allies both in Parliament and the party membership in general. Long-term supporter Nadine Dorries has argued he should return, as he received a mandate from the British public in the 2019 election.
Penny Mordaunt
In 2019, Penny Mordaunt made history by becoming the UK's first female defence secretary - a natural fit for a naval reservist who had already served as armed forces minister under David Cameron.
Previously, she had been best known outside Westminster for her appearance on ITV's celebrity diving show Splash! to raise money for charity.
MP for Portsmouth North since 2010, Ms Mordaunt is a former head of the Conservative Party's youth wing and was a press officer for William Hague when he was party leader.
She stood in the last Conservative leadership contest, and secured strong support from her fellow MPs but just missed out on making it the final two.
After backing Ms Truss, she was appointed leader of the House of Commons and Lord President of the Privy Council - which meant she presided over the Accession Council for the new king.
Ben Wallace
While many leading Tories divide opinion within the party, the defence secretary is widely seen by fellow MPs as a safe pair of hands.
Mr Wallace has drawn increasing attention since Russia's invasion of Ukraine, as the UK made an early decision to support Kyiv with weapons and training.
Despite his opposition to Brexit, Mr Wallace has been a key supporter of Boris Johnson and was rewarded with a cabinet post in 2019.
Before becoming a politician, he served as a soldier in Germany, Cyprus, Belize and Northern Ireland, where he thwarted an IRA effort to carry out a bomb attack against British soldiers.
After Mr Johnson stood down, there had been suggestions Mr Wallace could run - particularly as he was topping polls run by the Conservative Home website - however he opted to support Ms Truss instead, describing her as "authentic".