The FTSE 100 has fallen sharply as the easing of lockdown restrictions in England failed to assuage growing global fears about COVID case numbers and disruption.
European and US indices joined London's leading share index in plunging at Monday's open.
The FTSE 100 was 2.3% or 163 points lower on the day at the close of trading - led by falls of more than 6% for British Airways owner International Airlines Group (IAG) and ITV as investors fretted over the prospects for global travel and advertising revenues respectively.
Companies from aircraft engine maker Rolls-Royce and Holiday Inn to Crowne Plaza owner Intercontinental Hotels Group and Whitbread, owner of Premier Inn, were also among the worst hit.
There were no stocks in the risers' column for much of the day.
The declines, in cash terms, represented a loss of £44bn in the collective market value of the index's constituents.
Major European markets suffered greater percentage losses, with Spain's IBEX losing more than 4%.
The S&P 500 in New York was 1.6% down in late morning dealings.
Meanwhile the pound dipped by more than half a cent against the US dollar to just below $1.37, its lowest level in five months.
Analysts have pointed to rising COVID-19 cases globally as well as increasing fears about growing inflation as being behind the latest cautious turn for investors.
Susannah Streeter, senior investment and markets analyst at Hargreaves Lansdown, said of the plunge: "Far from giving investors a jolt of confidence, Freedom Day has seen it evaporate, as sharply rising infection rates disrupt businesses across the UK.
"From retail to manufacturing and hospitality, the warnings are coming thick and fast that mandatory isolation is leading to reduced business operating hours, a drag on sales and a reduction of output.
"Amidst concerns that soaring infection rates could derail the recovery are worries about inflation heating up, and the knock on effect of rising interest rates combined with the roll back of mass bond buying programmes."
In the UK, hopes for another leap back to normality for the economy as social distancing and mask wearing rules are abolished have been undercut by a resurgence in cases and warnings to the public to remain cautious.
Businesses are increasingly feeling the impact of large numbers of workers being absent because they are forced to self-isolate after being alerted by the COVID-19 app.
In the travel sector, fading hopes for holiday sales were dealt a further blow over the weekend as Britons returning from France were told they will still have to quarantine even if they are fully vaccinated.
Michael Hewson, market analyst at CMC Markets, said: "There was a great deal of optimism over the summer reopening, but as we look at how Delta variant infections are rising, some of that optimism is dissipating."
Britain's currency is in focus as investors watch for the outcome of restrictions being eased even as cases are soaring.
"The world will be watching the UK experiment with huge interest," Deutsche Bank strategist Jim Reid said in a note to clients.
"It could show a pathway back towards normality or it could be a warning to even heavily vaccinated countries that COVID will be a problem for a decent length of time still."