Some 110 business leaders have warned U.K. Prime Minister Boris Johnson of dire consequences if he fails to sign a trade deal with the European Union before the Brexit transition period ends in six months.

Chief executives, founders and directors from a swathe of small- to medium-sized companies, including Argus Media Group and Ebookers, said in a letter Britain has reached “the point of no return for needing to agree an EU trade deal and prepare the country for that by the end of this year.” The deadline for the premier to seek an extension to the transition period was June 30.

The signatories cited U.K. government analysis showing that leaving the bloc without a trade deal could see GDP shrink by about 8% over the next 15 years. The Chemicals Business Association and Manufacturing Technology Association also signed the message, which points out rising trade tensions between countries like the U.S. and China could make striking trade deals even harder.

Given the “enormous economic shock” from the pandemic, “a no-deal outcome would be hugely damaging to the U.K. economy,” they wrote. “We are therefore calling on you to ensure full transparency over the substance of these negotiations and these deals, to enable proper assessment of the potential economic consequences and through that assure us that the economic downside on our businesses can be minimized.”