Johnson told parliament in a resignation speech on Wednesday, after leaving the government last week, that he opposed the new plan agreed by May and her cabinet at Chequers.

"It is not too late to save Brexit. We have time in these negotiations," he said.

"We must try now, because we will not get another chance to do it right."

Johnson thanked May for her "courage and resilience," saying he was honoured to have worked with her on promoting her vision of a post-Brexit "global Britain," which May launched in the Lancaster House speech.

"I thought it was the right vision then, I think so today," he said. "But in the 18 months that have followed, it is as though a fog of self-doubt has descended."

Johnson told May that if she "can fix that vision ... she can deliver a great Brexit for Britain, with a positive and self-confident approach that will unite this party, unite this house, and unite the country as well."

May has been rocked by several resignations of eurosceptics from her government and did not immediately respond to Johnson's speech.

In parliament earlier, she defended her latest Brexit proposals after opposition lawmakers and some Conservatives said she had "caved" to eurosceptics in the party.

May's government narrowly won a vote late Tuesday on an amendment to its new trade bill that was presented by pro-EU Conservatives, who had sought to keep Britain in the EU customs union if no trade agreement is reached before it leaves the European Union in March.

During Wednesday's prime minister's questions, May also faced pressure from pro-Brexit Conservative Andrea Jenkyns, who asked: "At what point was it was decided that Brexit means Remain?"

May replied that "Brexit continues to mean Brexit," saying she was seeking a "workable solution ... to ensure we leave the European Union and embrace [a] bright future."

Australian Associated Press