Business and labor unions say the UK is facing a 'national emergency'

London(CNN Business) British business and labor unions have set aside their differences to make a joint appeal to Prime Minister Theresa May to change course on Brexit or risk a massive economic shock.

Top officials at the Confederation of British Industry and the Trades Union Congress said Thursday that Britain faces a "national emergency" if politicians allow the country to crash out of the European Union without a transitional deal to protect trade.

"Firms and communities across the United Kingdom are not ready for this outcome. The shock to our economy would be felt by generations to come," CBI boss Carolyn Fairbairn and TUC chief Frances O'Grady wrote in a letter to May.

UK labor unions and business lobbies rarely speak with one voice, but they have been united by fears that a disorderly Brexit would wreak havoc on the economy. That could happen in just eight days.

"We cannot overstate the gravity of this crisis for firms and working people," Fairbairn and O'Grady warned the prime minister.

They urged May to delay the Brexit deadline beyond March 29. They also recommended that the prime minister abandon the deal she has negotiated with the European Union -— rejected twice by the UK parliament — and start over.

May is asking the European Union to delay Brexit until June 30. EU officials have indicated they may be willing to accept a short delay if May can get her deal through the UK parliament next week. If that doesn't happen, the chances of the country crashing out of the bloc increase.

A disorderly exit would result in new costs and trade barriers for companies in Britain. The Bank of England has said the fallout from that scenario would be worse than the 2008 financial crisis.

Companies have been spelling out the risks for months.

McDonald's (MCD) and KFC (YUM) joined with UK supermarkets to warn that crashing out of the European Union would disrupt supplies.

Automakers are canceling planned investments, and preparing to idle factories. Airbus (EADSF) says that it would be forced to redirect future investment away from the United Kingdom.

Banks and other finance companies are moving assets worth at least £1 trillion ($1.3 trillion) out of the country.