In his resignation letter, Sir Alan described Brexit as "a dark cloud".
He told the BBC he quit to demand an emergency Commons debate to give MPs a chance to say whether they supported Mr Johnson's "wish to form a government".
The request for a debate - which would not constitute a binding no-confidence vote - was rejected by the Speaker.
Mr Johnson is the frontrunner in the contest which has seen him go head-to-head with Jeremy Hunt for Tory Party members' votes.
The ballot closes at 17:00 BST - the winner will be revealed on Tuesday morning and will become prime minister on Wednesday. » | Monday, July 22, 2019
