Tilda, one of the UK’s most popular rice brands, has said some of its business is at risk if the country withdraws from the customs union and single market.

The company’s joint managing director Umesh Parmar said the theory favoured by Brexiters that short-term pain was worth the sacrifice for the long-term gain of free trade deals around the world was misguided.

“For us, the pain would be to our business and people,” Parmar said. He said the company’s rice mill in Rainham, Essex, would shut if barriers to imports from its suppliers in India and Pakistan as well as exports to the EU were put in place.

Tilda employs 250 people and exports 30% of milled rice at the 24-silo plant in Rainham and its nearby ready-meal plant.

Founded in 1970 by the Thakrar family, who arrived in the UK from Uganda, Tilda started out with a small milling facility in Leicester. The company has grown and is currently worth £86m.

The issue for Tilda is that 30% of their products are exported to Europe on zero tariffs but, post-Brexit, this could rise to £145 a tonne.

Its Rainham-produced ready meals would be classified as non-British because rules of origin stipulate that the products cannot contain more than 20% UK content to remain on a zero tariff.

In the UK, there is also a potential tariff on ready meals, meaning there is no point in milling the raw or cargo rice in the UK.

Parmar said: “The Tilda brand won’t disappear, but it just won’t be a British-made product anymore. The value added will not be in Britain. Our business would be better off within the customs union.”

Alex Waugh, the head of the Rice Association, said flour mills were also at risk as 80% of Ireland’s bread flour came from the UK.

Post-Brexit tariffs could be up to 50% on flour imports from the UK, making British flour uncompetitive and threatening several of the country’s 50 mills.

• This story was amended on 15 May 2018. An earlier version gave an incorrect figure for the amount of rice that Tilda exports, and has been removed.