The only bright spots on the blue-chip index were Flutter Entertainment and Tesco.
The FTSE 100 was down 1.2pc by 0752 GMT. The index is headed for its worst day since Aug. 14, when the U.S. Treasury bond yield curve inverted for the first time since 2007, and raised fears of a recession.
The more-domestically focussed FTSE 250 fell 0.8pc.
The main index, falling for a third consecutive day, is looking at its sharpest weekly drop since May as the UK market faces not only worries over growth due to the U.S.-China trade dispute, but also mounting uncertainties at home over Brexit.
Johnson will present on Wednesday his final Brexit offer to the European bloc and said that, if Brussels does not engage, Britain will stop talking and leave on Oct. 31.
Analysts largely expect Brussels to reject the offer, leaving open options ranging from a no-deal departure to another delay.
Flutter Entertainment, formerly known as Paddy Power, jumped 13pc to top of the main bourse after agreeing to an all-share deal with Poker Stars to create one of the world's biggest online betting and gambling companies.
That lifted shares in its midcap rivals William Hill and GVC by 3pc and 5pc, respectively.
Tesco, Britain's No. 1 retailer, rose 2pc after better-than-expected earnings. Shares had opened lower after the surprise resignation of its Chief Executive Officer Dave Lewis.
"Retail bosses are like football managers, a handful of names always moving around the top jobs, notching maybe a season or two of success before inevitably failure catches up with them.
Many fail from the get go ... Tesco just lost Fergie (Alex Ferguson)," Markets.com analyst Neil Wilson wrote.