So who is in the running to become the next British Prime Minister? There are three top contenders.
Rishi Sunak
The former chancellor, who was runner-up to Ms Truss in the Tory leadership race, is favourite with the bookmakers to replace her.
He warned his rival that her tax-cutting plans would send the economy into free fall, accusing her of “fairytale economics” as she promised unfunded tax cuts.
Penny Mordaunt
The leader of the Commons finished in third place in this summer’s Tory leadership race before she backed Ms Truss’s bid.
Ms Mordaunt was sent to the Commons on behalf of Ms Truss to answer an urgent question on the sacking of former chancellor Kwasi Kwarteng on Monday.
Ben Wallace
There’s Ben Wallace – The defence secretary and former soldier is widely respected for the role he has played in the UK’s support for Ukraine.
He stayed neutral in the Tory leadership race before eventually backing Ms Truss.