A furious row over unions and press freedom has erupted again here and half the national papers burst into righteous indignation this past Friday morning.

In Fleet Street eyes, a great principle has been assaulted by the unions - an editor's right to print or omit what he pleases.

The battle was touched off by David Astor, who has just sold his loss-ridden Sunday Observer to the Atlantic Richfield oil company. Writing in Index, a distinguished monthly review of censorship around the globe, Astor says that newspaper unions are a more "lethal" threat to press freedom even than Britain's inhibiting laws.

He describes how printing unions here engage in passive and active sabotage, not only to enforce bargaining demands but even to suppress news of their own activities, such as squirting oil on paper or planting chewing gum on presses to break a run.

Astor explains that near-monopoly newspapers in the provinces (or the United States) can stand up to these tactics and defeat or break their perpetrators. But in Fleet Street's competitive cockpit, with eight daily national newspapers fighting each other for readers and ads, the loss of a day's production is costly and could be fatal.

Both The Times of London and The Guardian thought Astor's piece was newsworthy and prepared articles on it for their Thursday editions. The Guardian ran four pungent paragraphs on page nine but added, at the insistence of their mechanical unions, a last paragraph that said no such dreadful things had ever happened to The Guardian.

The Times, which had planned a longer article for page one, refused to follow suit. Editor William Rees-Mogg promised the protesting printing unions that he would normally publish a rebuttal statement or letter but would not add or subtract from the paper's copy that day. So, two unions shut him down and The Times lost the entire run for Thursday.

The affair, said Rees-Mogg, "made Mr. Astor's point about union censorship for him. Union censorship is a grave threat to press freedom."

But even the highly traditionalist British printing unions know that all newspapers are not paragons of uninhibited truth.

John R. Commons, the distinguished American labor historian, once said that unions take their style, color and attitudes from the employers with whom they bargain. The censorship imposed by printing unions here could be interpreted as another face of their bosses.

There is a vast difference in style between most British and American papers. British papers generally follow a 19th century tradition that makes less distinction between news and comment, that selects and displays stories to fit a philosophical preconception. Except for The Financial Times, a special case, the play and treatment of any story in the seven other national newspapers here is highly predictable.

For example, a top front-page story in Friday's Times features an attack by Margaret Thatcher, the Conservative leader on Prime Minister James Callaghan for "upholding censorship of the press."

The third paragraph, in a story bylined "Hugh Noyes, parliamentary correspondent," asserts that Callaghan made a "weak defense of press freedom." That judgment would be inadmissible in most American newspapers, except in an article clearly labeled as "analysis" or "opinion."

A few months ago, Noyes began his lead story in the paper this way: "Mrs. Thatcher last night brought Conservative MPs to their feet cheering with a burst of morale-boosting oratory which tore apart the government's economic strategy and left the chancellor grinning like a Cheshire cat before he disappeared."

The U.S. tradition would prohibit all the value judgments in that paragraph except in a column or editorial.

In the summer of 1973, Rees-Mogg wrote an editorial that accused the Ervin Committee, Watergate grand jury and U.S. newspapers of engaging in "lynch law" to bring down President Nixon. The paper's editors then buried reports from its Washington correspondent which provided material that did not fit that line.

Printing unions read and know about these things. They take editors' and publishers' protestations about freedom of the press with a large dose of salt - just as Prof. Commons might have predicted.