There are “increasing numbers” of Tory MPs speaking up for keeping close economic ties with the EU, including remaining in the customs union, the former Conservative Chancellor said.
“The first rule of politics is that you have got to learn to count,” said Mr Osborne.
On the growing Conservative revolt, he said: “That is going to pose a challenge to the Government, of course, but it is also going to empower Parliament.
“Last time I checked one of the principle arguments from the Brexiteers is that they wanted more parliamentary sovereignty.”
Mr Osborne argued the economic harm from Brexit had been laid bare in this week’s leak of the secret Government analysis – with the greatest damage forecast from pulling out of the EU’s economic structures.
“I question whether there is really a government majority for withdrawing from the customs union, now you have seen the economic figures from the Government,” he added.
The analysis predicts staying inside the EU’s economic structures would reduce growth by 2 per cent over 15 years, compared with current projections, according to the leak,
However, a no-deal Brexit, leaving Britain trading with Europe on World Trade Organisation terms, would reduce growth by 8 per cent – four times as much.
Mrs May wants a transition period which would effectively keep the UK in the single market and the customs union for about two years, to avoid a damaging “cliff edge” on Brexit day in March 2019.
But the Cabinet and the Tory party is split down the middle on what happens next – with the faction led by Boris Johnson demanding a clean break from the EU at that point.
Meanwhile, up to 20 Tory backbenchers are ready to vote with Opposition parties to keep the UK in the customs union permanently – with Labour still undecided.
Mr Osborne, who was sacked as chancellor by Mrs May, has described her as a “dead woman walking” and repeatedly used the pages of his London Evening Standard newspaper to attack her Brexit plans.
He told BBC Radio 4’s Today programme: “We now face a series of choices about the kind of Brexit we want and we have a much clearer idea of the consequences.
He added: “We should look clearly at the costs and benefits of, for example, leaving the customs union and doing less trade with Europe versus what we might gain from doing a trade deal with America.
“At the moment, the sums don't stack up for that kind of decision.”
However, Mr Osborne said he was not among those people who believed Brexit could yet be halted, perhaps in a further referendum.
Instead, he has called for the UK to remain within the European Free Trade Area (EFTA) – a form of soft Brexit, involving effective single market membership – alongside Norway and Switzerland.
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