Registrations fell to 942,340 cars last month from 1.03 million a year earlier, the consulting firm said on Monday. Its numbers are based on national sales data and estimates for some of the smallest markets.

Prospects for 2019 are "a little brighter", the firm said, predicting Western European sales growth of 0.8 percent for the year on the assumption that London and its EU partners reach agreement on the terms of Britain's departure from the bloc.

But a no-deal Brexit in March would likely lead to a 7 percent full-year UK sales decline, the forecaster said, pushing regional auto market growth into "negative territory" for 2019.

The December slump, steeper than November's 8.3 percent drop, came as problems with model availabilities eased following chaotic implementation of new "WLTP" test standards, which had dented business over the three previous months.

"There appears to be other issues impacting on sales," LMC analyst David Oakley said - citing weaker consumer confidence, Brexit concerns and France's "yellow jacket" street protests, which have disrupted business and weighed on demand.

The seasonally adjusted annualised rate (SAAR) of Western European sales also fell 3.7 percent from November to 12.84 million cars last month, LMC said.

Registrations fell 6.7 percent year-on-year in Germany, 14.5 percent in France, 5.5 percent in Britain and 3.5 percent in Spain, while Italy rose 2 percent, according to data published in recent days.

(Reporting by Laurence Frost; Editing by Toby Chopra)