Sterling could tank to as low as $1.15 as political divisions deepen, an analyst warned Wednesday.

British Prime Minister Theresa May’s leadership is under intense scrutiny from a number of disgruntled members of her own party — not least former Foreign Secretary Boris Johnson.

Stephen Isaacs, chairman of the investment committee at Alvine Capital, warned that the British pound could tumble to $1.15 — which represents a decline of more than 10 percent from current levels — by early 2019 if U.K. lawmakers on both sides quash May’s Brexit deal.

Isaacs said that the final test for the U.K.’s exit from the EU — and May’s leadership — will be a vote in the British parliament. He said the U.K. opposition Labour Party would likely exploit this “open goal” and vote it down, adding that he expected several euroskeptic lawmakers from May’s Conservative Party to follow suit.

“The prime minister has to resign, her key policy will be in absolute tatters,” Isaacs told CNBC’s “Squawk Box Europe.” He said May’s ousting won’t lead to a general election “because turkeys don’t vote for Christmas,” referring to her party’s withering majority in 2017’s snap election.

“You get an election for leadership of the Conservative Party, and I think Boris (Johnson) will be prime minister, that’s my case,” he said. Johnson on Tuesday told the Conservative Party’s annual conference that May’s current Brexit deal is “politically humiliating” and a “constitutional outrage,” in a speech that was viewed by observers as a potential leadership challenge.

Isaacs compared the possibility of May stepping down to resignations as notable as that of former British prime ministers Neville Chamberlain and Anthony Eden.