The currency initially fell for a third day running during morning trade, plunging a further 0.4% against the dollar to reach a 27-month low. Investor jitters center around concern that Johnson could pull the U.K. out of the European Union without a formal deal in place.
But immediately after Johnson was announced as the country's next leader, the pound pared its losses to trade around the flatline at $1.2474. Johnson's rise to power had largely been priced in by the markets and the announcement meant one rung of uncertainty had been removed.
Johnson will deliver his maiden speech as prime minister outside 10 Downing Street on Wednesday. On Tuesday, Johnson struck a compromising tone, promising to "unite the country," but reiterated calls for optimism on the prospect of the U.K. leaving the bloc.
Michael Brown, a senior analyst at Caxton FX, said his focus would quickly switch to the next steps: "Namely, Cabinet appointments and the Brexit plan. The latter will be of more importance for markets, with sterling set to remain under pressure should Boris continue his 'do or die' Halloween Brexit stance."
Britain's National Institute of Social and Economic Research (NIESR) published a report on Monday which suggested that there is now a 40% chance of a "no-deal" Brexit on October 31, an event anticipated by many to be profoundly damaging for the British economy.
Johnson has vowed repeatedly to take the U.K. out of the EU with or without a deal in place, while also rejecting the existing Withdrawal Agreement negotiated by his predecessor Theresa May, and pledging to return to Brussels to seek new terms.
In the past week, as 160,000 Conservative Party members cast their ballots to elect the country's next leader, both Johnson and rival Jeremy Hunt hardened their stance on the Irish backstop clause insisted upon by Brussels, therefore increasing the likelihood of a no-deal departure.
In a recent note, Berenberg senior economist Kallum Pickering raised the risk of a hard Brexit — in which the U.K. exits both the EU's customs union and its single market to trade on World Trade Organization terms — to 40%, making it Berenberg's base case.
However, as Pickering pointed out, Johnson has a reputation for adapting his rhetoric to changing tides of political sentiment. A Conservative member of the House of Lords, Michael Heseltine, once described Johnson as "a man who waits to see the way the crowd is running and then dashes in front and says 'follow me'."
Speaking to CNBC Monday, Pickering said an unresolved Brexit was like a "kidney stone lodged in the abdomen of the British economy," and projected greater sterling weakness until a resolution is found.
In a further note Tuesday, Pickering suggested that Johnson's promise to scrap the Irish backstop from the Withdrawal Agreement may just be a high-risk negotiating strategy, with a low chance of success, to push for compromise from Irish Prime Minister Leo Varadkar to accept a half-way deal. If the EU refuses Johnson's demand, Pickering predicted, moderate British lawmakers could move to thwart a no-deal exit, setting the stage for a "major showdown in Parliament" in the fall. This could lead to a further Brexit extension, a snap election or a second referendum.
"While Johnson may be forced to take a more pragmatic line eventually, we do not expect him to use soft words on Brexit in his first speech," Pickering wrote on Tuesday.
"In the not-unlikely event that he doubles-down and appears to harden his Brexit stance further, U.K.-oriented equities and sterling would likely react negatively."
Two further developments suggest that the hard Brexit risk has increased. Ahead of his anticipated coronation, Johnson has been surrounding himself with hardliners likely to box him into pursuing the exit door on Halloween come what may. As he begins to name his Cabinet this week, the extent of the euroskeptic makeup of his ministers could further impact the currency.
Meanwhile, the main opposition Labour party is sliding in the polls, rendering it a lesser threat in the event of a snap general election and potentially leading moderate Conservative members of parliament (MPs) to back no deal, in order to protect their seats from the surging Brexit Party.
The odds of an election in the fall are rising sharply. Stephen Gallo, European head of FX strategy at BMO Capital Markets, said in a note Monday that Johnson will have an "incredibly narrow window of opportunity" to exploit Labour's vulnerability, cut a deal with the Brexit Party to preserve Conservative seats, and lay out a post-EU and domestic policy agenda.
"That window starts to close rapidly by the end of the year, and there is no telling what 2020 will bring without a new parliamentary arithmetic for the Tory (Conservative) leadership to work with," Gallo said.
"We still believe there is more GBPUSD weakness to come."
Kamal Sharma, director of G-10 FX strategy at Bank of America Merrill Lynch, said the combination of Brexit factors weighing on the U.K. economy meant a potential "flash crash" for sterling could not be ruled out.
"The current account deficit has been the Achilles heel for the U.K. for a number of years now. Historically, the U.K. has been a very big recipient of FDI (foreign direct investment)," he told CNBC's "Squawk Box Europe" on Monday.
"It's now becoming more of a net debt story, so if investors start to suddenly give up on the U.K., for example, given the liquidity situation of sterling already, that really opens us up to a potential flash crash."
However, not all analysts were quite so pessimistic. Giles Keating, senior advisor at Torchwood Capital, told CNBC Tuesday that most of the bad news is already priced into sterling and investors could "start to look forward."
"What do you look forward to? Fiscal expansion — can the Bank of England react to a big fiscal expansion by cutting interest rates? It doesn't seem to me to make sense. That debate could end up being to hold rates here at a time when others were cutting them," he told CNBC's "Squawk Box Europe."
"The Bank of England is looking at an economy where wages are accelerating. The latest wage rises are really moving up sharply, we had the National Institute warning yesterday of 4% inflation as a risk — the Bank, in my mind, can't cut against that background."