LONDON (BLOOMBERG) - Boris Johnson, the former British foreign secretary, attacked Prime Minister Theresa May's "dithering" over Brexit negotiations, dismissing her plan as "enforced vassalage" as Britain remains under some European Union rules.

Johnson, a thorn in May's side when he was in office, used a parliamentary convention to explain his decision to resign from government 10 days after quitting. While praising May's "courage and resilience," he tore apart her Brexit strategy.on

"We dithered and burned through our negotiating capital," Johnson told lawmakers Wednesday (July 18).

"Worst of all, we allowed the question of the Northern Ireland border to dominate the debate."

"This is Brexit in name only," Johnson said of the plans that May agreed with most of her Cabinet at her country retreat called Chequers.

"It is not too late to save Brexit."

After 18 months of stealthy retreat we've come from the "bright certainties" of May's Lancaster House speech to the "miserable permanent limbo" of the Chequers agreement, he said.

FACE OF BREXIT

The former London mayor was the face of the Brexit campaign in 2016.

In his resignation letter, he argued that the prime minister's latest plan was a signal to the Brexit-voting public that the referendum was being betrayed by too soft an exit strategy from the EU.

The Brexit "dream is dying", he said, and Britain was heading for the status of a "colony."

When a photo shoot was staged for Johnson's signing of his resignation letter, murmurs of leadership ambitions swirled again.

Donald Trump on his visit to Britain last week said Johnson would make "a great prime minister."

Johnson's words are being closely watched, because of historic precedents. In 1990, for example, Margaret Thatcher's deputy prime minister Geoffrey Howe's barbed resignation speech over Britain's relationship with Europe prompted another member of her Cabinet to launch a leadership bid.

Johnson, who spent two years as foreign secretary under May, was noted for his gaffes abroad.

His former Cabinet colleague David Davis quit hours before Johnson last week.

Since then, he has given multiple media interviews in which the former Brexit secretary criticised her approach to negotiations but backed her leadership.

Johnson, meanwhile, had not made any public statement on May's leadership since he took the decision to go. In a column in

Monday's Daily Telegraph, he limited himself to appealing for a more "positive" view of Britain.'s prospects after Brexit.