London's main bourse, which makes a lion's share of its income abroad, fell 0.9 percent and the more domestically-focussed FTSE 250 was 0.7 percent lower.
Yet markets briefly trimmed their losses after US President Donald Trump predicted the United States would reach a deal on trade with China.
The FTSE 250 broke a six-day winning streak, as markets await a vote on British Prime Minister Theresa May's Brexit deal on Tuesday that is likely to decide the next phase of Brexit proceedings.
"If the vote does not pass this week, the magnitude of defeat will be important to determine the government's next move," BlackRock's strategist Richard Turnill said.
"A second referendum or a general election may yet be required to break the deadlock."
May urged lawmakers to take "a second look" at her deal, warning them that voting it down could open the way for the breakup of the United Kingdom.
Homebuilders, among the most exposed to concerns about a cooling economy amid Brexit uncertainty, slid despite a double upgrade on the sector from JP Morgan. They fell 0.2 percent after hitting their highest since November.
CHINA WORRIES RE-EMERGE
Investors also dumped stocks they deemed more exposed to China where data showed that December exports fell from a year earlier in their biggest monthly drop in two years.
HSBC was among the biggest drags on the FTSE 100 with a 1.3 percent drop, while miners tumbled 1 percent in response to signs of weakness in the world's top metals consumer.
Gambling firm Paddy Power dropped 4.1 percent to the bottom of the index, while retailer Next fell 2.3 percent after broker downgrades.
Among midcaps, Premier Oil tanked 12 percent after the Sunday Times reported that it was considering a cash call to fund its $1.5 billion bid for some of Chevron's assets.
In response, Premier Oil said no decision had been taken regarding the bid or how any deal would be financed.
Recruiting firm PageGroup slumped 7.1 percent after flagging that Brexit uncertainty was continuing to impact confidence, pulling down peers Robert Walters, SThree and Hays.
JD Sports was a ray of sunshine in a battered retail sector with a 6.4 percent jump after upbeat guidance.