Michael Bloomberg, the former mayor of New York | Jim Watson/AFP via Getty Images Michael Bloomberg: UK leaving customs union is ‘indefensible’ UK legislators are ‘squabbling over the choice between a bad result and a terrible one,’ billionaire and ex-New York mayor writes.

British Prime Minister Theresa May's promise to take Britain out of the EU customs union after Brexit is "politically understandable, yet entirely indefensible," ex-New York mayor Michael Bloomberg writes in an op-ed published today.

Staying in the customs union "would leave the U.K. with a worse deal than the one it currently enjoys as a full member of the EU" and be "toxic" to Brexit supporters, the billionaire says. But crashing out of the union without a deal in a so-called hard Brexit would leave the country even worse off, he writes in Bloomberg Views.

Leaving the customs union is a central pillar of the government’s Brexit strategy because it would allow the U.K. to pursue a trade policy independent of Brussels. Senior ministers are due to meet next week to discuss the issue ahead of a key House of Commons vote that could put pressure on May over the U.K. government’s decision to exit the union.

A hard Brexit would "throw Britain's trade relations into disarray and inflict grave harm on the economy" as well as "jeopardize peace in Northern Ireland," Bloomberg writes.

Staying in the customs union would help mitigate economic fallout from Brexit and help resolve the border issue. As such, May's refusal to seriously consider it "is indefensible," he writes.

"The Brexit vote was a mistake, and ought to be reversed now, not later," he added. "Britain's members of parliament are mostly opposed to Brexit, yet can't bring themselves to do their jobs and act on that conviction. The country and its legislators are therefore left squabbling over the choice between a bad result and a terrible one."