Big business is already suffering the consequences of Brexit, according to a survey of senior executives from FTSE 500 firms.
The Ipsos Mori captains of industry research showed more than half (58%) feel the decision to leave the European Union has had a negative impact on their firm since June's referendum.
Less than a third (31%) feel Brexit has made no difference to their business and just 11% thought it has had a positive impact.
The survey comes amid a potential backbench Tory rebellion over Prime Minister Theresa May's threat to leave the EU with no trade deal if exit negotiations collapse without an agreement.
Pro-Remain Conservatives are seeking assurances Parliament will get a say on the "endgame" amid fears the UK could pull out with no deal, with potentially serious consequences for the economy.
Despite signs of economic resilience following the Brexit vote, two thirds (66%) of business leaders surveyed think their company's situation will worsen over the next year, with only 13% believing the decision will have a positive impact.
In the long-term, bosses are more optimistic, with around a third (32%) anticipating a positive impact in five years time, but with 45% still expecting a hit to their firm.
They delivered a warning to Mrs May about the importance of the exit talks, with 84% agreeing or strongly agreeing "how well the Government actually handles Brexit negotiations is vital to my business".
Half (50%) said they were not confident in the Government's ability to negotiate the best possible deal for British business.
But the vast majority (96%) of executives were confident their firm can adapt to the consequences of leaving the EU, although one in 10 (10%) said they were taking action to move business outside the UK.
Their two most important priorities for the negotiation were securing the movement of labour and access to skilled labour (54%), as well as securing free trade or retaining the single market with the EU (47%).
Mrs May has said she will end the free movement of all labour, leave the single market, and seek to strike a new free trade agreement with the EU.
:: Ipsos Mori conducted 114 interviews with respondents from the top 500 companies by turnover and the top 100 by capital employed in the UK.
Respondents were chairmen, chief executive officers, managing directors/chief operating officers, financial directors or other executive board directors.