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March is generally the top-selling month of the year as it is one of only two months when new licence plates are issued.
But after record-high sales in 2015 and 2016, full-year demand dropped in 2017 due to uncertainty about future environmental levies on diesel cars and weaker consumer spending after June 2016's Brexit vote, which pushed up prices.
Demand was also artificially inflated in March last year, when customers brought forward purchases to beat a tax rise which came into force in April 2017.
The Society of Motor Manufacturers and Traders will release full data at 0800 GMT.