British Prime Minister Theresa May’s former foreign secretary Boris Johnson has warned that her Brexit strategy spells disaster for the country.

“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 September 3. “People can see Chequers means disaster.”

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, criticising the plan for regulatory alignment.

As reported by the Reuters news agency, May is struggling to sell what she calls her business-friendly Brexit to her own party and across a divided country.

The prospect that she could fail to reach a deal that would carry parliament at home, and that Britain could potentially crash out of the EU in March with no deal in place at all, has worried financial markets.

The plan, named for the prime minister’s country residence where it was agreed 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.

According to Reuters, 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.

“If we let the British pick the raisins out of our rules, that would have serious consequences. Then all sorts of other third countries could insist that we offer them the same benefits. That would be the end of the single market and the European project,” Johnson wrote.

However, May’s spokesman said the Chequers deal was the only credible and negotiable plan for Brexit and the government believed it could carry the support of parliament.

“There’s no new ideas in (Johnson’s) article to respond to. 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 spokesman told reporters.