After less than two years as prime minister, Keir Starmer announces his resignation amid mounting pressure from within his own Labour Party.Nearly two years after leading the Labour Party to a landslide election victory, Prime Minister Keir Starmer has announced his resignation amid a mounting rebellion within the party, kick-starting the race for the seventh prime minister of the United Kingdom in 10 years.In an emotional address on Monday, Starmer said he would remain in office until a new Labour leader and, by extension, the next prime minister is selected. The formal leadership contest is to begin on July 9 and is to be completed by the UK Parliament’s summer recess.Weeks of internal pressure after disappointing local election results had already weakened Starmer’s position. A decisive parliamentary by-election victory in Makerfield by challenger and Greater Manchester Mayor Andy Burnham ultimately triggered the resignation.Who has led the UK over the past 10 years?Since 2016, the UK has had six prime ministers, averaging one every one and a half to two years, compared with the much longer tenures of Margaret Thatcher and Tony Blair, who each led the country for more than a decade.The graphic below shows the 10 prime ministers the UK has had over the past 50 years.(Al Jazeera)