The number of cars manufactured in the UK fell 11% last month, the Society of Motor Manufacturers and Traders (SMMT) said.
The trade body said the slide compared with July 2017 was due to model changes, seasonal adjustments and preparation for stricter new emissions standards.
A total of 121,051 cars were produced in July.
That was down from 135,954 cars in the same month last year.
Last July was a particularly strong month, with the launch of several new models boosting output by almost 10,000 units. It also resulted in a 17.7% rise in domestic demand, the SMMT said.
Production of cars for export also fell in July by 4.2%, but overall for the year to date, exports only dropped by 1.2%, and continued to account for just over 80% of output.
"While the industry is undoubtedly feeling the effects of recent uncertainty in the domestic market, drawing long-term conclusions from monthly snapshots requires a health warning," said SMMT chief executive Mike Hawes.
"The bigger picture is complex and month-by-month fluctuations are inevitable as manufacturers manage product cycles, operational changes and the delicate balance of supply and demand from market to market.
"To ensure future growth, we need political and economic clarity at home, and the continuation of beneficial trading arrangements with the EU and other key markets."
source: BBC News