Former UK foreign secretary Boris Johnson. Photo: Reuters/Jeff J Mitchell

When Brexit Secretary David Davis resigned overnight, following a breakthrough deal reached by UK Prime Minister Theresa May, there was immediate speculation as to whether others would follow suit.

Should Foreign Secretary Boris Johnson follow Davis out the door, some speculated, it would be the fatal blow for Britain’s already bruised and battered Prime Minister.

It didn’t take long for that question to be answered. News broke on Monday afternoon, London time, that Johnson has resigned, marking the third cabinet member to do so over May’s approach to Brexit.

Johnson reportedly now thinks that no Brexit deal would be preferable to the so-called Chequers plan, for which May secured support on Sunday.

The British pound fell sharply versus the dollar and euro on the news, before recovering some of its losses.

His departure marks the first serious threat to May’s position as Prime Minister. Johnson is seen as a potential challenger to May, and his resignation means that there is a realistic possibility that she will not survive a vote of confidence in Parliament.

The development also calls into question the path forward for Brexit. The deal May reached on Sunday was a breakthrough that represented a practical and achievable compromise. Johnson’s resignation signals that the door to such a soft Brexit is closing. Others, including European Commission President Donald Tusk, are hoping the door will soon be shut for Brexit altogether.

The fate of May’s rule, the conservative party’s coalition government and Brexit will in all likelihood be decided in fresh general elections, which would be the third in just five years.