The London top flight index has plummeted nearly 9% since the start of the year as it dropped 2.7% in afternoon trading.
The FTSE 100 Index was down 158.7p to 5,689.3, as global markets continued to endure a turbulent start to the year, triggered by the falling oil price and a slowdown of growth in China.
Germany's Dax and the Cac 40 in France were both down more than 3%, while Brent Crude fell just under half a US dollar to 33.69 US dollars a barrel.
Oil prices have fallen more than 70% since the summer of 2014, as oil-producing nations in the Middle East have kept up production - even though demand has eased - in a bid to pressure US shale oil and gas suppliers.
BP fell just under 2%, while Royal Dutch Shell was down just under 1%.
Embattled engine-maker Rolls-Royce saw its share price fall more than 2%, as reports speculate it could chop a quarter off 2014's payout of 23.1p a share.
The company – which has been dogged by profit warnings – will reveal plans for its dividend on Friday when it lays bare its full-year results for 2015.
Global advertising firm WPP fell 5%, after broker Investec said slowing global growth threatened ad revenues.
Broadcaster ITV also saw its shares fall 7%, as investors marked its shares down for the same reason.
Apple chip maker ARM Holdings was almost 6% lower after smaller rival Imagination Technologies said conditions in the semiconductor industry have "not improved" since its last update in December.
Shares in ARM have been under pressure since the US technology giant Apple last month forecast its first drop in revenues since 2003 later this year, due to slowing iPhone sales.