UPDATE 1-China data hits UK shares ahead of Tuesday's vote on Brexit deal
London's main bourse, which makes a lion share of its income abroad, was down 0.6 percent and the more domestically-focussed FTSE 250 was 0.8 percent lower as at 0941 GMT, both on course for their worst day since Jan. 3.
Midcaps 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.
The agreement is expected to be rejected and could lead to a phase of uncertainty and volatility just as the fourth-quarter earnings season kicks off in earnest in the United States.
Homebuilders, among the most exposed to concerns about a cooling economy amid uncertainty over Brexit, slid despite a double upgrade on the sector from JP Morgan.
They were 0.2 percent lower after hitting their highest since late November.
CHINA WORRIES RE-EMERGE
Investors also dumped stocks they deemed more exposed to China after data from China showed December exports fell 4.4 percent from a year earlier in its biggest monthly drop in two years.
HSBC dropped 1 percent to be the biggest drag on the main index, while miners tumbled in response to signs of weakness in the world's top metals consumer.
Although luxury stocks fell on the weak data, Burberry defied the trend with a 2.1 percent rise thanks to a Bank of America Merrill Lynch upgrade.
Gambling firm Paddy Power and retailer Next fell nearly 3 percent each after downgrades by brokers and were the FTSE top losers.
Among midcaps, Premier Oil dropped 10 percent after the Sunday Times reported that it was considering a cash call to fund its $1.5 billion bid for NYSE-listed oil giant Chevron .
In response, Premier Oil said no decision had been taken regarding the bid or how any deal would be financed.
Recruiting firm PageGroup slumped 6.4 percent after a trading update in which it flagged 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 8.6 percent jump after it gave an upbeat guidance. (Reporting by Muvija M and Shashwat Awasthi in Bengaluru; editing by Josephine Mason and Jon Boyle)