The CBI's factory order book balance fell to +4 in March from +10 in February, its lowest since October and below all forecasts in a Reuters poll of economists that had only pointed to a modest drop to +9.
However, the CBI said March's reading remained well above the long-run average of -14 for the series, and the export order balance held steady at +10.
"Robust global growth and the low pound have gifted UK manufacturers a strong first quarter in 2018," CBI economist Anna Leach said.
Britain's agreement this week of a transitional deal with the European Union, which will ensure trading arrangements are little changed immediately after it leaves the bloc in March 2019, should help business, Leach added.
"Confidence among manufacturers will have been given an additional boost by the agreement of a transition deal, giving them the confidence to continue investing and planning for growth," she said.
(Reporting by David Milliken, editing by Andy Bruce)