Conservative MP Boris Johnson walks through his garden at his home near Oxford, Britain, October 3, 2018. REUTERS/Toby Melville

LONDON (Reuters) - Brexit hardliner Boris Johnson, a former British foreign minister, said on Wednesday that Prime Minister Theresa May’s negotiation strategy could make Britain a “permanent EU colony”.

Johnson, who resigned over May’s Chequers proposals and is seen as a contender to one day lead her Conservative party, criticised Britain’s strategy over the “backstop” - a fallback option for the Irish border should Britain and the EU fail to negotiate terms on the future trading relationship.

“Clearly No. 10 are negotiating a ‘backstop’ that makes the UK a permanent EU colony. We cannot escape EU laws & (European Court of Justice) until they allow us to – which they may never do,” Johnson said in a tweet, referring to May’s office at Number 10 Downing Street.