London — Prime Minister Theresa May’s Brexit strategy means disaster for Britain, former foreign secretary Boris Johnson said, as critics at home and officials in Brussels stepped up their opposition to her plans for how to leave the EU.

With less than two months before Britain and the EU want a deal to end over 40 years of union, May is struggling to sell what she calls her business-friendly Brexit to her own party and across a divided country.

May’s former deputy, Damian Green, described her government as "walking a narrow path with people chucking rocks from both sides" after Johnson, a potential successor to May, and other Conservative MPs attacked what is called the "Chequers plan".

"In adopting the Chequers proposals, we have gone into battle with the white flag fluttering over our leading tank. If we continue on this basis we will throw away most of the advantages of Brexit," Johnson wrote in the Daily Telegraph newspaper on Monday. "People can see Chequers means disaster."

The plan, named for the prime minister’s country residence where it was agreed to by the cabinet in July, calls for free trade between Britain and the EU in manufactured and agricultural goods, with Britain accepting regulations over traded goods that align with EU rules. The government says it is the only way to achieve Brexit without harming the economy.

But opponents on both sides of the Brexit debate have criticised it for offering either too sharp a rupture with the EU or a break that is not clean enough.

Johnson, one of the leading pro-Brexit campaigners during the referendum that secured Britain’s 2016 vote to leave, quit May’s cabinet days after the Chequers plan was approved.

"We will remain in the EU taxi; but this time locked in the boot, with absolutely no say on the destination," he wrote on Monday, criticising the plan for regulatory alignment.

May’s spokesperson said the Chequers proposals are the only credible and negotiable plan for Brexit and the government believes the plan could carry the support of parliament.

" What we need at this time is serious leadership with a serious plan, and that is exactly what the country has with this prime minister and this Brexit plan," the spokesperson told reporters.

The Times newspaper reported 20 Conservative MPs are now backing a grassroots "StandUp4Brexit" campaign, committing to opposing the Chequers plan.