Natural gas supplies may be shut off to industrial users in the eastern half of the United States through the end of February unless a "substantial additional supply" of gas can be made available to pipelines serving the East.

This is the blunt warning of the Federal Power Commission's Bureau of Natural Gas, which has calculated that 200,000 to 300,000 workers have been idled by gas shortages caused by the prolonged cold wave that has stricken the region as far south as Florida.

Although natural gas is still available to some industrial users, depending on the pipelines that serve them, industrial supplies are beingsharply curtailed and can no longer be guaranteed even for firms without the capability to convert to other energy sources, according to government and industry officials.

Natural gas normally constitutes roughly half the fuel that goes into industrial production in the United States and about 30 per cent of the nation's total energy supply, according to FPC officials.

Most large industrial employers have the capacity to switch to other sources, such as coal, oil, propane or expensive synthetics, and many have done so, they said. But they added that many smaller plants have not prepared to convert and may be forced to close for a matter of weeks.

"No one can look you straight in the eye and say they didn't know this was coming," said Paul Feine, special assistant to the executive director of the FPC. "But they didn't think it was going to be quite this bad."

Feine estimated that some but "not many" workers will be laid off for more than a week or two.

The weather outlook is not encouraging. After a two-day break in the cold wave, the National Weather Service yesterday issued winter storm watches for Indiana, southern Illinois, northern Kentucky, northeastern Kansas, southeastern Iowa and most of Missouri - embracing some of the border-state areas that suffered severely from earlier chills and resulting gas shortages.

The warning of at least five more weeks of industrial gas supply suspensions, contained in a memorandum to the commission on Friday underscores the urgency behind President Carter's appeal later in the day for Americans to turn thermostats down to 65 degrees or lower and for emergency legislation to help channel more gas into the fuel-starved East.

It also puts increased pressure on industrial firms to convert as soon as possible to other fuels. However, government and industry sources say some of these alternates, such as propane, also are in short supply because of cold induced distribution difficulties.

And, in addition to industries not currently equipped for conversion, some industries such as those involved in production of glass, fertilizers and some food products require natural gas for at least some of their process.

"Some - theres no way of knowing how many at this point - will simply shut down and not reopen for the rest of the winter," said an FPC official.

According to the staff memo and to FPC officials monitoring the gas pipeline shortages, the government's primary effort has been to assure adequate supplies to residential customers and commercial users providing vital services.

Industrial users have had to be cut off or severely limited even though the resulting plant closures go against the new administration's avowed goal of reducing unemployment and stimulating production.

"It has been necessary to curtail innumerable industries with or without alternate fuel capabilities along with schools and some commercial establishments," the memorandum stated. "This has led to considerable unemployment and economic loss. With the exception of New England, the whole area east of the Mississippi has been affected to varying degrees by these plant shutdowns."

Emphasizing that "vital services" have first priority, the staff memo asserted that "not until this is assured can industrial services be renewed," and added pointedly:

"It appears that the eastern U.S. pipelines may be unable to serve any requirements other than residential and commercial through February. A substantial additional supply would be necessary before all firm industrial requirements could be met."

The memorandum added that a reduction in residential and commercial usage would "free up considerable volumes for use by industries with no alternate fuel capabilities." Voluntary conversion to other energy sources by industries with such a capability would have the same effect, it noted.

Meanwhile, at the White House during the second day of receptions celebrating Carter's inauguration, maintenance men trooped through the rooms turning down thermostats and affixing them with signs saying: "Please keep thermostats at 65 degrees - The Management."

Seated with National Security Council members around a crackling fire in the Cabinet room fireplace, Carter said, "This is the last warm meeting we'll have."