The British government today put a $5 billion price tag on what will be the world's largest single public offering of stock shares -- the sale next week of 50.2 percent of British Telecom, this country's government-owned telephone company.

The sale is the centerpiece of conservative Prime Minister Margaret Thatcher's five-year campaign to turn over large chunks of British industry that have been in government hands to private enterprise in the hope they will become more efficient and competitive.

The Telecom sale, however, dwarfs any previous British sale and also vastly overshadows the $1.4 billion offering in 1971 by the American Telephone & Telegraph Co. that is believed to be the current record holder.

Britain's minister for information and technology, Geoffrey Pattie, today announced in Parliament that the price per share had been set at the equivalent of $1.64 each. This means that the three billion shares to be offered should bring in about $4.9 billion to the British treasury. The government will retain control over the remaining 49.8 percent of the firm.

The announcement of the share price brought immediate denunciations by opposition Labour Party spokesmen, who have consistently criticized each move to denationalize a government-run industry.

Labour spokesman Alan Williams accused the government of selling a highly profitable asset, which he said will earn some $1.6 billion this year, out from under all of Britain's taxpayers and handing it over "at a knock-down price to a relatively small portion of those people."

Critics have argued in the case of Telecom -- as well as in the recent sale of the profitable Jaguar car division of British Leyland and the earlier sales of British Aerospace and Britoil -- that the government is selling off the most profitable firms and leaving the less efficient divisions or companies to remain permanent public money losers.

Nevertheless, the government argues that firms already denationalized show they perform still better in private hands, and that it also reduces government borrowing, spurs innovation and initiative and leads to better management and wider ownership.

Pattie said 14 percent of the shares will be offered on stock exchanges in New York, Tokyo and Toronto. There is considerable global interest in the sale and its aftermath among managers of other government-owned systems in Europe and in Japan, which may put its state-owned system up for sale next spring.

Telecom is the world's fourth-largest telephone company. But unlike the huge AT&T operation in the United States, it has no competition here. With some 241,000 employes, Telecom will automatically become Britain's largest single private company and employer.

The prospectus for the sale, which is expected to be heavily oversubscribed, will be published next Tuesday, with the last date for applications to buy in minimum lots of about $312 set for Nov. 28. Actual trading begins on Dec. 3.

The government has spent massively on an advertising campaign to create a positive attitude toward the sale, including sweeteners such as offering rebates of about $23 on telephone bills for each minimum lot bought and held for eight months to those among Telecom's 16 million phone customers. It is estimated the cost of underwriting the offer and advertising it may have hit some $240 million.

The price announced today is said by London stockbrokers to be on the high side of the range hoped for by the government.