LONDON (AP) _ The government plans to prosecute a sixth British newspaper for defying its ban on publishing sections of ''Spycatcher,'' the memoirs of a former British intelligent agent.

Britain on Tuesday obtained a High Court order in New Zealand barring a Wellington newspaper from printing any more parts of the book by Peter Wright.

Editors of the British paper, the News on Sunday, and the Australian paper, the Wellington Dominion, said they would fight the restrictions.

The left-wing News on Sunday said it received a letter from government lawyer Sir John Bailey saying the attorney general authorized the start of criminal contempt proceedings.

The newspaper published a 700-word extract detailing Wright's allegations that members of the British MI5 counterespionage agency plotted to destabilize Harold Wilson's Labor government in the 1960s and 1970s.

Britain's highest court, the Law Lords of the House of Lords, last week upheld a media ban on ''Spycatcher.'' The government says Wright violated his pledge to maintain silence about his experiences and compromised national security.

The Law Lords also barred reporting of Wright's allegations made during a court action in Australia, where Britain is fighting to prevent publication of the book.

News on Sunday editor Brian Whitaker said his paper will publish further material from Wright's book on Sunday.

''It is unacceptable that in a democracy like ours the British press should not be allowed to publish stories concerning this country which are appearing in other newspapers throughout the world,'' Whitaker said.

Five other British newspapers face prosecutions for publishing excerpts from the book, which is selling briskly in the United States, Canada and other countries. Under British law, a publisher found guilty of contempt can be fined and jailed.

In New Zealand, a spokeswoman for Attorney General Sir Patrick Mayhew said:

''The government obtained an ... injunction today in the High Court of New Zealand telling the Wellington Dominion to prevent the publication of further extracts from 'Spycatcher.'''

Geoffrey Baylist, editor of the Dominion, accused the British government of ''draconian censorship.'' The newspaper published excerpts from ''Spycatcher'' on Tuesday.

''South Africa has very harsh press laws but they are allowed to report what happens in an open court,'' Baylist said in a British Broadcasting Corp. radio interview. ''Yet here, the British newspapers are not able to do it. It is quite incredible.''

Britain took no legal steps to stop the publication of ''Spycatcher'' in the United States. It sent a letter to Canadian publishers Stoddart Publishing, however, threatening to sue for any profits made.

On Saturday, Britain obtained an interim injunction against Hong Kong's biggest English-language newspaper, South China Morning Post, which published excerpts the previous week.

Book store owners in Hong Kong said today that initial orders of ''Spycatcher'' had sold out and they had long waiting lists of customers seeking the book.

''Demand is considerable, and I'd say 70 percent of it is due to the controversy. Everyone wants to be the first on the block to have it now,'' said one downtown book seller, who spoke on condition of anonymity.

Legal experts in Hong Kong said the injunction against the South China Morning Post probably did not apply to book store sales, but book sellers said they did not advertise ''Spycatcher'' in their windows because they feared legal problems.