Mike Hawes, SMMT chief executive, said: "With the country locked down in crisis mode for a large part of March, this decline will come as no surprise".
"How long the market remains stalled is uncertain, but it will re-open and the products will be there".
"Such a dramatic decline in registrations is proof that COVID-19 will ricochet through the industry for weeks and months ahead", said Karen Hilton, chief commercial officer of vehicle buying website heycar.
The figures also reveal the impact that lockdown procedures have had on other countries' new auto markets. In total, 6,818 were registered last month.
With 203,370 fewer cars registered last month versus March 2019, it was actually the worst March for sales since the late nineties, the SMMT said.
The figures, due out at 0900, are expected to show a drop of more than 40% compared with a year ago.
That means battery-electric cars overtook plug-in hybrids in popularity, with PHEVs seeing a more modest 38 per cent increase in sales. In the first half of the month we see the impact of the new '20 United Kingdom license plates in an environment of heightened consumer confidence and dealer optimism - March usually drives sales in H1.
They fell to the lowest level seen in March for more than two decades.
Michael Woodward, Deloitte's head of automotive, said: "The industry is beginning to introduce flexible measures around payments to protect the interests of consumers".
Sue Robinson, director of the National Franchised Dealers Association (NFDA), said it remained "crucial that the Government continues to support businesses in the automotive sector, one that employs over 590,000 people and is a key contributor to the United Kingdom economy".
"In the second half though we see the effects of Covid-19 and the United Kingdom lockdown on the motor industry".
Carwow chief executive, James Hind, believes that there are signs that the vehicle retail sector remains more positive than March's dramatic shift into sales lockdown indicated by the SMMT data might suggest, meanwhile.
"It will be the used vehicle market that bounces back quicker with more buyers looking for reliable used or almost new models, and cars being returned from PCP agreements and part-exchanges entering back into the market".
It is a popular time to buy because it is one of two occasions when new number plates are released.
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