FILE PHOTO: Britain's former foreign secretary Boris Johnson walks to Downing Street in London, June 26, 2018. REUTERS/Toby Melville/File Photo

LONDON (Reuters) - Brexit hard-liner Boris Johnson, a fierce critic of Prime Minister Theresa May’s “Chequers” plan for leaving the EU, declined to answer directly on Friday when asked if he would rule out a leadership challenge of the Conservative party.

Asked in an interview by BBC political editor Laura Kuenssberg whether he would rule out challenging May, he replied:

“My job is to speak up for what I believe in and the vision that I have set out today and I believe in it very, very sincerely and I’m going to keep going for as long as it takes.”

Former foreign minister Johnson, the bookmakers’ favorite to succeed May, said earlier on Friday that her Brexit plans would leave the United Kingdom half in and half out of the club it joined in 1973 and in effective “enforced vassalage”.