British commercial vehicle (CV) output fell 28.3% year on year to 6,132 units (vans, trucks, buses and coaches) in April 2017 as the four-day, late Easter 'bank holiday' affected production.
Year to date output also declined, down 8.1% to 29,583 units.
Exports continued to account for the majority of production in 2017, up 12.6% to 19,087 (the highest since 2010) though April shipments fell 10.5% to 4,027.
The SMMT noted the year on year comparison data reflected very high levels of demand over the past two years and fleet purchasing cycles affecting the timing of monthly production volumes.
The proportion of CVs built for export markets reached 64.5% for the first four months of the year.
Mike Hawes, SMMT chief executive, said: "With fewer working days in April and following two years of exceptionally strong demand, it's not surprising to see a decline in CV output, especially when you consider the effect of fleet buying cycles on manufacturing timing.
"However, as the proportion built for export reaches levels not seen since the start of this decade, it highlights the importance of maintaining good links with our biggest trading partner, Europe. Securing a strong trading relationship will be vital to the continued success of UK CV manufacturing."