"If she is unpunished, then the opposition party will put down a vote of no-confidence in the government," the British politician told CNBC's Martin Soong at the UBS Wealth Insights conference in Singapore.
But a no-confidence vote does not mean a general election, so May "is likely to come back and try to put here proposal, perhaps in an amended form, a few days later," he predicted. That could potentially lead to "a whole series of indicative votes" on different options for the country's future, he said.
"There is a long way to go and sadly, this is a prolonged period of uncertainty for investors and the economy but also for the political system. "
Failure to back Tuesday's deal" would be a catastrophic and unforgivable breach of trust in our democracy, "May wrote in a Sunday Express editorial over the weekend. The vote was originally meant to take place in December but it was delayed on concerns or insufficient approval from the U.K. Parliament and May's Conservative Party, with disapproval from both Brexiteers and Remainers.