Vague and unpredictable Trump administration immigration policies threaten the US economy amid a tightening labour market, CEOs from major American companies have warned.In a letter to Department of Homeland Security Secretary Kirstjen Nielsen yesterday, the Business Roundtable urged a rethink of recent directives from Washington that have thrown the status of workers into doubt and are damaging the recruitment of talent."Few (workers) will move their family and settle in a new country if, at any time and without notice, the government can force their immediate departure -- often without explanation," the roundtable said in the letter. "At a time when the number of job vacancies are reaching historic highs due to labour shortages, now is not the time restrict access to talent."The Business Roundtable represents most of the biggest US companies. The group has criticised the Trump administration before on immigration, while praising the White House for cutting taxes and regulation.Among those chief executives signing the letter: Apple's Tim Cook, JPMorgan Chase's Jamie Dimon, Coca-Cola's James Quincey, IBM's Ginni Rometty and Marriott's Arne Sorenson.The executives faulted US Citizenship and Immigration Services, which is under Nielsen's authority, for having inconsistent determinations that can result in sudden denials to employees who have been successfully permitted for years.In other cases, the department has revoked work permits to spouses of legal immigrant workers. In some cases, these problems affect employees permitted under at the H-1B program, which lets firms hire skilled foreign recruits to fill specialized positions."Out of fairness to these employees -- and to avoid unnecessary costs and complications for American businesses -- the US government should not change the rules in the middle of the process," the letter said.