London's main bourse, which makes a lion share of its income abroad, was down 0.4 percent and the more domestically-focussed FTSE 250 were both down 0.4 percent at 0850 GMT.
The agreement, which May and EU leaders say cannot be renegotiated and is the only one available, will almost certainly be rejected. If so, uncertainty, paralysis and the likelihood of a disorderly 'no deal' Brexit will rise.
In a speech to factory workers in Stoke-on-Trent in central England, Prime Minister Theresa May is expected to say that lawmakers blocking Brexit is now a more likely outcome than Britain leaving the European Union without a deal.
Homebuilders, among the most exposed to concerns about a cooling economy amid uncertainty over Brexit, hit their highest since late November in early deals as JP Morgan handed the sector a double upgrade to overweight, following an upgrade by BAML last week.
HSBC dropped 1.3 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 as expected on the weak data, Burberry defied the trend with a 2 percent rise, outperforming FTSE 100, thanks to a Bank of America Merrill Lynch upgrade of the stock.
Gambling firm Paddy Power and retailer Next were the top losers on the main index after downgrades by brokers.
Investers were nervous ahead of fourth-quarter earning season which kicks off in earnest in the United States this week.
Among midcaps, recruiting firm PageGroup slumped 4 percent after a trading update.
But JD Sports was a ray of sunshine in a battered retail sector with a 7.4 percent jump after saying annual earnings would be at the upper end of market view following a strong Christmas period. (Reporting by Muvija M and Shashwat Awasthi in Bengaluru; editing by Josephine Mason)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)