The FTSE 100 has been knocked after oil prices slumped during Donald Trump's historic meeting with Russia's Vladimir Putin in Finland.
Brent crude prices were down by as much as 3.75% to 72.002 US dollars per barrel as traders anticipated the outcome from the Helsinki summit.
BP and Shell fell by 2.3% and 2.1% respectively, and mining stocks were also knocked, with BHP Billiton falling by 2.48% or 41p to 1,609.2p.
The FTSE 100 closed down 0.8% or 61.42 points at 7600.45.
"A sharp slide from Brent Crude left the FTSE far and away the worst performer on an otherwise somnambulant start to the week," said Connor Campbell, financial analyst at SpreadEx
"Down 3% as investors fret over what will be discussed between Trump and Putin, namely whether or not the US president will encourage Russia to up its oil production once more, Brent crude found itself in trouble the wrong side of $73 per barrel."
European indices put in a mix performance, with the Cac 40 in France falling by 0.36% and the Dax in Germany up by 0.16%.
The pound was flat against both the dollar and the euro in late afternoon trading, although currency traders are gearing up for more activity during the week with data on jobs, inflation and retail sales coming out over the next three days.
Debenhams' shares fell out of fashion again on Monday following reports that credit insurers have reduced cover for suppliers to the struggling department store chain.
Credit insurance company Euler Hermes is among those to have pulled some cover, while Atradius and Coface have refused to cover new shipments, according to the Sunday Times.
Shares were knocked by 4.8% or 0.71p to 14p. The news comes after Debenhams issued its third profit warning this year in June.
Shares in pharmaceuticals group Indivior rocketed after a US court imposed a preliminary injunction on a rival firm selling or importing a generic version of one of its key drugs.
A US District Court in New Jersey granted the restriction against Indian pharmaceutical company Dr Reddy's Laboratories from selling its treatment of opioid addiction, sending Indivior's shares up 16.9% or 49.3p to 340.5p.
Shares in the John Laing Infrastructure Fund (JLIF) also soared on Monday after the firm said it was likely to accept a £1.45 billion takeover offer from a consortium of investors.
JLIF, which invests in public infrastructure projects across Europe, has been approached by Dalmore Capital and Equitix Investment Management.
By the close, JLIF's shares were up 18.1% at 21.4p to 139.6p.
Shares in Finsbury Food Group closed 1.5p higher at 116.5p, despite the company facing declining full-year revenue.
The cake and bread manufacturer said that revenue fell 3.4% to £303.6 million in the year to June 30, blaming a challenging environment and rising costs for the drop in sales.
However, its core UK bakery division grew 2.8% on a like-for-like basis in the period, ahead of the wider market.
The biggest risers on the FTSE 100 were Ocado up 25p to 1,054.5p, DS Smith up 6.1p to 496p, CRH up 27p to 2,718p and Barclays up 1.84p to 191.52p.
The biggest fallers on the FTSE 100 were Micro Focus International down 37p to 1,241.5p, BHP Billiton down 41p to 1,609.2p, Severn Trent down 46p to 1,895p and BP down 13.3p to 557.5p.