Brexit has shattered the U.K.'s image as a place for investment, the former chairman of one of the country's biggest banks said Monday. A second referendum on whether the U.K. should leave the EU may be inevitable, Gerald Grimstone, who served as chairman of Barclays, warned ahead of crucial Parliament votes this week on Prime Minister Theresa May's Brexit deal. And he didn't mince his words. "Brexit is completely unpredictable for the U.K. ... I think it's been an unmitigated disaster for the U.K., and in the short term, it's made the U.K. almost uninvestable," Grimstone told CNBC's Dan Murphy at the Global Financial Forum in Dubai Monday. "We have two contrasting scenarios before us, and nobody knows which way we're going to go."

On a knife's edge

Only 18 days before the U.K.'s scheduled departure from the EU on March 29, the prime minister is trying, and so far failing, to get concessions from EU leaders on the exit deal before it goes to a vote on Tuesday. This will be the second vote on the deal, after it suffered a punishing defeat in January. If a simple majority of them don't approve the deal, they are then due to vote on whether they want to leave the 28-member bloc without a deal. If this is also refused, they'll then have a vote on whether to extend Article 50 (which sets out the departure process) and delay Britain's departure.

May and other Brexit officials have tried to allay concerns of U.K. lawmakers with reassurances from the EU on key Brexit issues like the "Irish backstop". But EU officials have so far offered May little relief, with both sides expressing increasing frustration at what they see as continued deadlock.

Game over for Theresa May?

Some politicians and business players have said that time is up for May, who's already survived two votes of no confidence from lawmakers in her own party and from the opposition. The former Barclays chair stressed the urgent need for the U.K. to make up its mind on what Brexit will actually look like. "I've got huge respect for PM May, but they didn't get the politics right," he said. "The country is going to have to make a choice at some point, either in the single market, or out."

It's time for a drastic change, Grimstone said — and that may mean a second public vote on Brexit, something that many "remain" lawmakers, as well as several business leaders, have called for. "I think a second vote is almost unavoidable," he said. "That vote might come through a general election and/or a second referendum. You cannot have a scenario where the government of the day is putting legislation through the Parliament and that legislation fails... If a government can't get its legislation through Parliament, there will have to be a change of characters in the Parliament."

UK investment is taking a hit — and could suffer more