LONDON (AP) _ Telephone party lines and dial-a-date numbers are being banned in Britian, where critics say teen-agers may be too hung up on the costly social calls.

British Telecom, the telephone company, ended its own service a year ago and announced Friday that it was indefinitely suspending the other privately run ''chatlines'' effective Monday.

''We have taken this action in the public interest following acute and growing public concern over chatlines, which we feel has brought the whole service into disrepute,'' said a company statement.

Critics of the party lines - with which people can call a number and speak to like-minded strangers - welcomed news of the cutoff but expressed worry over those in debt to the telephone company.

Costs for the services were 66 cents per minute during peak hours and 44 cents per minute during off-peak times. Newspapers have printed stories of parents facing phone bills of $5,000 to $10,000 run up by their children.

Although British Telecom said it could not waive high bills ''we will work out a way in which an account can be paid which is best for the family,'' said Duncan Lewis, director of products and strategy.

The Independent Chatline Association, which represents most of the 100 companies providing the services, was holding emergency talks to consider the news.

British Telecom was privatized in 1984 but remains the country's main telephone company, with monopoly privileges and a near-monopoly on services.

A final decision on the party lines, which are enjoying wide popularity in the United States, is expected after the Monopolies and Mergers Commission publishes the results of a study on Feb. 28.