London's blue chip share index has tumbled deeper into the red amid a global stock market rout sparked off by woes in China and plunging oil prices.
The FTSE 100 Index slumped below the 6000 mark after diving by more than 140 points - a fall of 2.4% - in the wake of more heavy losses in Asia.
China's benchmark Shanghai Composite Index plunged more than 7% overnight - triggering the second so-called circuit breaker move to halt trading this week.
Markets in Asia are being hit by the withdrawal of Chinese government measures introduced last year to prop up share prices, while investors are also unnerved by signs of a worsening slowdown in China's economy.
Steep falls in oil prices are compounding the worldwide stock market sell-off, with the cost of benchmark Brent crude collapsing to fresh 11-year lows, sinking below 33 US dollars a barrel.
The picture was dire across Europe, with the Dax in Germany shedding more than 3% and France's Cac 40 off 2.5%.
Connor Campbell, financial analyst at Spreadex, said: "The global indices have been left in the midst of what is arguably their most calamitous week since the storied trading of last August."
Chinese stock trading was also suspended on Monday after a plunge that sent markets reeling in a dismal start to 2016.
The Shanghai benchmark has dropped 12% in the first week of the new year alone.
Today was due to mark the end of a six-month ban by Chinese regulators on sales by shareholders who own more than 5% of a company, but regulators announced earlier this week that sales would be limited to private transactions in an effort to calm volatility.
Traders fear the share sale ban first introduced last summer in China may have only delayed the sell-off.
It was first introduced as part of a panicked response by Beijing last summer, after Chinese markets nosedived by 30% as the country's stock market bubble burst.
The Shanghai benchmark had more than doubled between late 2014 and its June 12 peak.
Andy McLevey, head of dealing at stockbroker Interactive Investor, said: "With oil prices continuing to plummet and geopolitical tensions to the fore, investors are scurrying to the sidelines and even these current levels may not be enough to tempt many back short term as the uncertainty continues."
The latest stock market falls come as Chancellor George Osborne warns over a "dangerous cocktail" of threats to the UK economy in a speech being made in Cardiff.
He will say: "We are only seven days into the new year, and already we've had worrying news about stock market falls around the world, the slowdown in China, deep problems in Brazil and in Russia.
"Commodity prices have fallen very significantly. Oil, which was over 120 dollars a barrel in 2012, now stands at less than 40 dollars."
In London, commodity stocks led the declines, with blue chip oil companies among the hardest hit as the cost of crude continues to be hammered by global over-supply and falling demand amid the slowdown in the world economy and China.
BP and Royal Dutch Shell both fell more than 5%.
Marks & Spencer was one of only two stocks in positive territory, up 1% as the retailer announced the retirement of chief executive Marc Bolland in April and named 25-year M&S veteran Steve Rowe as his replacement.
The news came as the high street bellwether revealed a dire performance from its clothing division, with like-for-like general merchandise sales down 5.8% over the Christmas quarter, although its food halls saw robust growth of 0.4%.
M&S chairman Robert Swannell insisted there had been no shareholder pressure on Mr Bolland to quit, adding that his succession had been planned for years.
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