London — The jewel in Britain's culinary crown has long been a splendidly hearty breakfast - porridge, sausages, fried eggs, bacon, fried tomatoes, toast, marmalade, tea. The best way to eat here, as one critic was unkind enough to suggest, is to eat breakfast three times a day.
Humorist A.P. Herbert went further. Breakfast, he wrote in 1935, ''is the critical period in matrimony.''
But now the jewel, and the moment, is under threat as never before. American-style breakfast television has burst into British homes to a mixed chorus of applause, despair, and rejection, and sausages and fried tomatoes may never be the same again.
''How,'' asked one aggrieved commentator after the first breakfast TV program ever to be shown in Europe opened on the BBC at 6:30 a.m., Monday, ''can you eat a British breakfast with one hand while running from kitchen to living room to watch?''
Americans have long known how it's done. NBC's ''Today Show'' has been on the air in the United States for 31 years. Canadians, Australians, Japanese, and Hong Kong residents wake up to TV, and their anchor men (''presenters,'' as the British call them) sent cheery filmed greetings for the first BBC show. The television man in Tokyo bowed politely into the camera.
An American might be forgiven for wondering why the British have to run from kitchen to living room to watch TV at all. Americans have had small (or even large) TV sets in their kitchens for decades.
But Britain is different. Radio has dominated the early morning here, and only 40 percent of British homes have more than one TV set. Only a tiny fraction of second sets are in the kitchen.
The BBC has spent (STR)6 million ($9.6 million) gearing up its new 21/2 hour ''Breakfast Time'' program. Its commercial competitor ''TV-AM,'' which goes on the air Feb. 1, has spent (STR)20 million. Both are hoping that, as in America, the British will watch TV in the morning simply because it is there.
BBC officials say they will be happy if at any time during the morning, 2 million people are watching. They hope people rushing to work will watch for 15 or 20 minutes, and that the elderly, the unemployed (more than 3 million in these recessionary days), and others will watch for longer periods.
The BBC program showed itself Jan. 17 as a low-key, relaxed mixture of 20 -minute segments, each made up of news, weather, sports, regional slots, and feature interviews. It was rather like the NBC-TV ''Today Show'' with less haste , less noise, and a clipped accent.
Jane Pauley, NBC ''Today'' presenter for the last seven years, came over to appear as a guest, and to do her own program from London later in the morning when Americans were just waking up. The BBC team sat on red couches and tried hard to be more informal than the accepted BBC image. Weather and sports presenters wore no ties. The idea was to make viewers feel they were looking in on someone's home. The BBC even had coffee pots bubbling on camera in the background. A keep-fit expert tried to lead commuters at Waterloo Station in morning exercises.
When ''TV-AM'' comes along, it will be larger, splashier, and longer - 31/4 hours (6:00 to 9:15 a.m.) - and on weekends as well. It will star David Frost and Michael Parkinson as well as locally known Angela Rippon and others.
However, a number of problems remain for both programs. Overall TV viewing in Britain was down 6 percent last year. Some say it is because audience ratings are compiled in a new way, but others feel that the British public has better things to do. A union dispute is threatening ''TV-AM,'' (as well as the new Channel Four), with actors in TV commercials demanding more money and the makers of commercials refusing to pay up.
Judging from surveys, including a highly unscientific sampling of opinion by this correspondent on Jan. 17, the viewer verdict is not yet in.
One poll on the eve of the new program showed 72 percent of people not prepared to disrupt breakfast to watch TV.
''I turned it on because I was curious,'' said an office worker, ''but I won't watch again. I don't have the time.''
''I didn't watch because I woke up late,'' another office worker said. ''But I will tomorrow, just to see.''
''I like it,'' said an older woman in Surrey after her son had gone to work. ''It had a relaxed atmosphere. I liked the little clock face in the bottom right hand corner of the screen telling you the time all the time. And I like someone going through all the newspapers for you.''
''I'm against it,'' said a neighbor's wife whose three children had just gone to school. ''My husband wanted to watch though, and we have a TV set in the bedroom, so on it went. I prefer the radio.''
''Definitely not, no,'' replied an older woman in Wimbledon. ''We eat breakfast in the kitchen. We'd feel guilty watching TV in the morning. . . .