The FTSE 100 is expected to fall below the 6,000 point barrier for the first time since January 2013 on Monday as world stock markets continue to take fright over the state of China's economy.
Some Asian stocks fell to three-year lows on Monday following sharp sell-offs last week.
Monday saw the Shanghai Composite tumble 6.2% to 3,289 in early trading, having lost more than 10% already this month.
Hong Kong's Hang Seng index fell 4.2% to 21,475.
Japan's Nikkei 225 stock index dropped 2.5% to 18,956.
Australia slid 2.5% to 5,084.
South Korea's Kospi lost 0.5%.
Takako Masai, the head of research at Shinsei Bank in Tokyo, said: "Markets are panicking. Things are starting look like the Asian financial crisis in the late 1990s.
"Speculators are selling assets that seem the most vulnerable."
Qi Yifeng, analyst at consultancy CEBM, said: "The market is in a downtrend. There's no good news, stocks are still expensive, and there's no fresh money coming in."
The Chinese authorities have intervened numerous times to try and arrest the sell-off - the latest measure allowing the state pension fund to invest up to 30% of its assets in Chinese stocks.
Even before the Chinese markets opened on Monday, stocks across Asia took a beating.
Fears of a China-led global economic slowdown drove US stocks to their steepest one-day drop in nearly four years on Friday.
The FTSE 100 endured its worst week of trading of 2015, losing 5.5% by Friday. It meant that £93bn was wiped from the value of the index.
Financial spreadbetters expected the London market to open 3.5% lower on Monday - taking the FTSE 100 below the 6,000 point barrier.
