Leaving the EU could make the UK "critically short" of workers, with hiring intentions already hit by Brexit uncertainty, according to a new report.
Employment group Manpower issued a warning about the impact on British industry if EU workers were no longer able to move freely to this country.
Leaving the EU would be particularly damaging for top construction firms, which traditionally rely on European workers, and have been warning for months of skills shortages, said the report.
A survey of 2,100 employers showed the biggest fall in retail optimism in five years, prompted by the recent launch of the national living wage.
James Hick, managing director of the Manpower Group, said: "Britain added 404,000 jobs in the last 12 months alone, and despite the uncertainties of Brexit, employers tell us they still need more workers.
"Make no mistake about the vital contribution EU workers make to Britain. There are currently 2.2 million people from the EU working in the UK, but not all of them will stay here in the long term and we need the opportunity to replace the skills they bring.
"Britain today is a magnet for international talent, from finance to tech to the NHS. Leaving the EU will make it much more difficult to attract the brightest and best. It will mean more bureaucracy for those coming to Britain and salaries could be less competitive, especially if sterling falls, as many warn it could.
"To compete on the world stage British businesses need the flexibility and free movement that EU membership brings."
Employers in the West Midlands and Wales were most positive about taking on new staff, while the least optimistic were in Yorkshire and Humberside.