Boris Johnson has pulled ahead of his closest Tory leadership rivals with a slew of endorsements from prominent party moderates and Brexiters as outsider candidates warned against a “stitch-up” barring them from the ballot. Johnson, who has kept a low profile in the contest so far, launched his bid in earnest with a video posted on social media highlighting his campaigning credentials. His team then released endorsements throughout the day from MPs across the party’s spectrum, including the Brexit minister Kwasi Kwarteng and Simon Hart, who coordinates the moderate Brexit Delivery Group. Concern is growing among some Conservative MPs that a change in the rules could lead to a coronation of Johnson before some of the lesser-known candidates have had a chance to gain momentum. There are 13 declared candidates and the list could grow on Tuesday. Supporters of the defence secretary, Penny Mordaunt, say she will make a decision about running after an evening conference call with grassroots activists. The 1922 Committee of Tory backbenchers, which will run the contest, will hold an executive meeting on Tuesday as calls grow for changes to the rules to reduce the size of the field. The communities secretary, James Brokenshire, compared the…