Firms that rely on EU workers have warned of the "catastrophic" impact of proposals to slash unskilled migration on the day Britain leaves the EU.Under the draft plan, leaked to the Guardian, firms would have to recruit locally unless they could prove an "economic need" to employ EU citizens.
They could face a skills tax to boost training of UK workers if they still chose to employ unskilled EU staff.
But business groups say a "sudden" cut could cause "massive disruption".
The National Farmers' Union claimed the "entire food supply chain" could be threatened.
NFU deputy president Minette Batters said: "We are calling for an urgent and clear commitment from government to ensure that farmers and growers have access to sufficient numbers of permanent and seasonal workers post-Brexit.
"And we need clarity on the new rules for EU nationals living and working in the UK well before free movement ends in March 2019."
'Under control'
The leaked Home Office document has not been signed off by ministers, who will set out their post-Brexit migration plans later this year.
But Defence Secretary Michael Fallon said: "The public voted to leave the European Union. That means freedom of movement has to end."
He said "people with the right skills" would still be "welcome".
But he added: "Equally we have to make sure that British companies are also prepared to train up British workers.
Analysis
BBC political editor Laura KuenssbergThe prime minister has at least two big reasons for wanting to get this right.
For Theresa May, the referendum result was a clear instruction from the British people that they wanted to reduce the levels of immigration. Politically, therefore, she believes it's a demand she has to meet.
And as home secretary for six years, when the government continually flunked its own immigration target, the new system that will control immigration is finally, perhaps, a chance to meet her own long-missed goal.
So Wednesday's mega-leak from the Home Office of the potential design of the post Brexit system is significant. Read more
Simon Jack: Business sounds alarm
"The public are very clear, they want to see immigration not stopped but brought properly under control."
His message was echoed by Theresa May at Prime Minster's Questions, who told MPs immigration had to be cut to ease the strain on public services, adding that it "often hits those at the lower end of the income scale hardest in depressing their wages".
The EU has not issued an official response to the leaked document. Unnamed sources have told The Times the EU would block access to the single market during the transition period the UK wants after Brexit if it presses ahead with the proposals.
Michael Fallon said the government would take the views of business into account when drawing up its migration policy.
But business groups have hit back at his suggestion that they are using cheap foreign labour rather than training up British workers.
'Social needs'
The British Hospitality Association said: "If these proposals are implemented it could be catastrophic for the UK hospitality industry and for those who enjoy the hospitality it brings."
The BHA claims 75% of waiters, 25% of chefs and 37% of housekeepers in the UK are EU nationals and at least 60,000 new EU workers are needed every year to fill vacancies.
The organisation said it would take 10 years to train up enough British workers to plug the gap and some businesses would fail in the meantime, "taking UK jobs with them".