The FTSE 100 shed 0.8% and the mid-cap FTSE 250  fell 0.9% by 0743 GMT. Both indexes were on course for their first monthly falls this year, down 3.5% and 4.5% respectively since the start of May.

Data on Friday showed China's factory activity shrank more than expected in May, another stark reminder of the economic ramifications of the Sino-U.S. trade dispute.

Combined with Trump's Mexican standoff, that led London's indexes of financial stocks and miners to give up more than 1% each, while heavyweight oil stocks also skidded.

"The worry is who's next on Trump's list - the EU may be next," Markets.com analyst Neil Wilson said. "Coming at a time of a breakdown in talks with China, it's another blow to bulls and we should consider further downside risks from escalation."

Housebuilders fell after mortgage lender Nationwide said British house price growth had eased to its slowest in three months, pointing to the continuing impact of the Brexit process on consumer sentiment.

On the mid-cap index, Wizz Air stumbled despite forecasting net profit growth for the year ahead and saying it was well-placed to tackle higher fuel costs.

Shares of the low cost carrier fell 5.6%, with traders citing a marginal annual profit miss and rising fuel costs.

AIM-listed Stride Gaming surged 23% in light trading after it confirmed larger peer Rank Group was in advanced discussions to take over the company. Shares of Rank were roughly flat.