Negotiators were struggling to clinch an 11th-hour deal, raising the chances that Johnson will have to seek an extension of the Oct. 31 deadline for Britain's exit from the bloc.
Sterling has surged some 5% since late last week, when London and Brussels restarted intense negotiations.
But on Wednesday sources in the bloc said the talks had hit a "standstill".
Disagreements centre on a future trade deal and the rejection by Northern Ireland's Democratic Unionist Party (DUP), which supports the ruling Conservative Party in the British parliament, of customs arrangements tentatively agreed by negotiators for the Irish border.
The DUP's leader Arlene Foster dismissed an earlier report that her party had come round to an agreement.
"It looks like a deal is being worked on but everyone who works with the EU knows that these deals happen at the last minute," said Kit Juckes, head of currency strategy at Societe Generale in London. "It seems more likely than not that we will need an extension."
London-listed companies that make their cash at home, from housebuilders to banks, on Wednesday reversed some of the ground gained since last week.
These domestically focused stocks, some of the world's most unloved shares in recent years, have seen their fortunes transform since Friday - JPMorgan's domestic basket has outperformed London-listed exporter peers and the blue-chip FTSE 100.
Britain's mid-cap stocks index FTSE 250 cut its losses from the morning and was last down marginally. Ireland's main stocks index fell 0.5%.
"UK equities have started to look more interesting. We've been on the negative side for some time, (but) I wouldn't be so pessimistic there any more because valuations have started to look more attractive. A lot of bad stuff is priced in," said Legal & General Investment Management strategist Lars Kreckel.
"If the Brexit issue were to disappear or become less prominent, you could have some flows returning to UK equities."
British government bonds benefited from the renewed uncertainty, with 10-year yields broadly flat on the day at 0.706%.
With the spotlight on Brexit, September inflation data had little market impact. Britain's inflation rate failed to rise as expected last month as petrol prices fell at the fastest rate in more than three years, a boost to consumers ahead of Brexit.