SMMT figures also show that plug-in hybrid registrations were down by more than 40% in May.
Overall, the alternative fuel vehicle market rose by 12.7% in April, but zero emission-capable plug-in hybrids fell by 34%, says the SMMT.
The UK true fleet market suffered a 9% decline last month, delivering the worst March figures since 2014.
UK new car registrations declined by 3.4% last month, with fleet and business registrations falling by 3.9%, as political and economic uncertainty and continuing confusion over diesel affected demand.
The Tesla Model 3 led the BEV and premium mid-size saloon rankings, according to analysis from Jato Dynamics.
Fleet and business registrations in January were almost 5% (4.88%) lower than in the same month last year, according to the latest figures from the Society of Motor Manufacturers and Traders (SMMT).
The UK recorded the second highest true fleet sales in Europe last year, according to Dataforce.
Fleet sales were down more than 7% last year, as the new car market suffered its worst performance in five years.
UK car registrations have fallen behind the rest of Europe, with a 6.4% decline between January and June.
True Fleet registrations have grown by .02% in May, following the first rise in 13 months in April.
The UK true fleet market grew for the first time in 13 months in April, rising year-on-year by 6.3%, figures from Dataforce show.
New car registrations grew 10.4% in April, including a 0.9% growth in fleet.
True fleet sales in the UK have continued to decline, although February 2018 saw the lowest volume loss in a month-on-month comparison since February 2017.
The UK’s true fleet market fell 11% last month - the first January since 2012 it hasn't seen growth, according to Dataforce.
Britain’s True Fleet Market remained in a slump during October, representing the seventh consecutive month of decline.
Ford has maintained its position at the top of the UK vehicle sales rankings in May but the overall figure for registrations is falling.
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