SHANGHAI  Some of the world’s biggest corporations are facing intense pressure from China to allow the state-approved union to form in their Chinese plants and offices. But many companies fear admitting the unions will give their Chinese employees the power to slow or disrupt their operations and will significantly increase the cost of doing business here.

The companies, many of which moved to China to lower manufacturing costs and some to avoid unions in their home countries as well, are now being asked to meet a Sept. 30 deadline to make their offices and factories union shops.

Companies that do not comply risk being publicly vilified or blacklisted by the union, and perhaps penalized by the government, since businesses are required by law to allow unions to form.

Lawyers and analysts say that demands of the All China Federation of Trade Unions, the only union the Communist Party allows, could sharply alter business practices of foreign companies in China, including giving lower-level workers the power to bargain over anything from pay raises to whether a Chinese headquarters should be moved elsewhere in the country.