The FTSE 100 Index dropped 42.59 points to 6053 in early trading due to China’s slowdown, Brexit fears and commodity uncertainty.
The Shanghai Composite fell about 2.9% after the G20 meeting failed to come up with any specific stimulus plans for China’s ailing economy.
Back in the UK, a government analysis said a process of agreeing withdrawal and setting up new trade deals if the UK left the EU would hit financial markets, the pound and two million ex-pats.
But a Moody's downgrade of Oman's sovereign credit rating to A3 showed the economic pressure that Middle Eastern oil producers are facing from continued low oil prices.
"The key driver for the rating downgrade is the highly negative impact of the structural shift in lower oil prices, which has continued through the opening months of 2016, on Oman's government finances,
balance-of-payments position, and economic performance," the credit rating agency said.
GlaxoSmithKline's (LON:GSK) stock drifted 2.5p to 1408p on reports that the drug giant had started succession planning to replace chief executive Andrew Witty, although not until 2017 at the earliest.
Elsewhere, Corero Network Security (LON:CNS) sparked 10.1% to 24.5p as the cyber-security specialist won an order worth more than US$200,000 for its SmartWall Threat Defense System from an unidentified European regional supplier.
The FTSE 100 Index is poised to start the week lower as Asian shares fell overnight and after the G20 meeting ended.
The UK benchmark has recovered from some early year losses and made it above the 6,000 level again, closing Friday up 83 at 6, 096.
Today, it is tipped by spreadbetters at IG to open around 56 points lower.
The weekend G20 meeting finished with, according to one view at least, no firm plan for a fiscal stimulus to improve global growth, though there was a wider agreement to use all "policy tools" available.
Asian markets did not seem to react warmly. The Nikkei in Japan closed down 161 points at 16,027, while the Shanghai Composite Index is trailing 115 points at the time of writing.
In the US, the Dow Jones closed Friday 57 points lower at 16,640.
That came despite better than expected US data at the end of the week with consumer spending rising last month and inflation going higher by the most in four years. All this feeds further into the view will now raise US interest rates.
It is also causing strength in the US dollar.
The key focus in this release will be current trading and outlook comments, particularly given the disappointing January national advertising performance reported in the recent DMGT trading update, reckons broker Numis.