The United Kingdom could see a 15% rise in construction costs if it votes to leave the European Union, the head of London’s largest private homebuilder has predicted.

In a letter to clients, suppliers and corporate contacts, the managing director of Galliard Homes, Don O’Sullivan, said that he believed that construction companies in the UK would be hit by increased material prices and more expensive labour if the June 23 EU referendum vote led to a ‘Brexit’.

“If the UK is not part of the EU, then industry construction costs could rise by up to 15% since currently construction materials imported from and exported to the EU are free of duty and taxes,” O’Sullivan said in the company letter.

“Many site/construction staff working in London are people who originate from countries across the EU,” the letter continued, with Galliard also predicting that the London property market would likely falter as money and people leave the UK capital, a Construction Enquirer report added.

“This is clearly a big decision for all, with many competing factors and issues, but we at Galliard Homes strongly believe that it is better for the short and long term growth of the London economy, and the wider UK, for the country to remain within the EU,” O’Sullivan wrote.