An underground warren of offices known as "the bunker," where Winston Churchill and his Cabinet directed the fight against Nazi Germany while the Luftwaffe was bombing the streets of London above, is to be opened to the public next year. It offers fascinating tableaux of Britain's wartime leadership at work.

The Cabinet war rooms should be ready for tourists by next summer, officials of Britain's Environment Ministry said today, following a refurbishing that will cost as much as $3 million. The changes will leave the completely preserved premises and their contents essentially intact, including an inscription at Churchill's seat at the conference table bearing a quote from Queen Victoria: "Please understand there is no depression in this house and we are not interested in the possibilities of defeat. They do not exist." The present prime minister, Margaret Thatcher, cited the same quotation in April in voicing her determination to recover the Falkland Islands from Argentina.

Virtually nothing has been touched since Churchill left the complex for the last time nearly four decades ago when Japan surrendered. Wall maps reflect the progress of the fighting. One officer's sugar ration was found neatly wrapped in an envelope, and in Churchill's bedroom, from which he delivered many of his most famous wartime speeches, cataloguers noted a porcelain chamberpot and a humidor.

"It looks like everyone just went out at lunchtime for a beer," said Peter Simkins, a historian at the Imperial War Museum who is supervising the project. The remodeling will actually enhance the longevity of many of the rooms by sealing them with glass viewing walls to prevent deterioration.

The bunker, located under Whitehall, close to the prime minister's residence at 10 Downing Street and not far from Buckingham Palace, became operational on Sept. 2, 1939, the day before Britain declared war on Germany. When the rooms were appropriated some months earlier, they were storerooms with tunnels probably dating back hundreds of years. Behind one wall, a handsome musket dated 1800 was discovered.

The rooms were in use throughout the war, but particularly during the German blitz in the autumn of 1940 and during the V1 and V2 rocket attacks in the summer of 1944. An adjoining group of offices was to serve as the command center for home forces if the Nazis had mounted a land invasion of Britain.

The key office was called the map room. It operated around the clock for planners and intelligence experts to assess incoming information. Security was extremely tight. Records show that only 46 people were permitted access. Red, green and white telephones sit on the crowded tables, along with books, maps and lamps. In an annex next door were photo scanners and, among other things, a telephone switchboard for the bunker's number, Whitehall 5422.

Across the narrow corridor is perhaps the strangest room. From the outside it looks like a pay-toilet stall with a small lock on the door to show if it was vacant or occupied. The room was known as Churchill's personal lavatory, but its sole purpose was to provide complete security for the prime minister to telephone president Roosevelt. There is no plumbing inside, only a chair, a small table and a telephone with a clock on the wall set for London and Washington time.

Churchill's bedroom has a large desk, from which the British Broadcasting Corp. broadcast his speeches, and a bed. He rarely if ever slept in the room, although at least once, it was said, he put on his pyjamas and dressing gown before deciding at the last moment that he preferred his Downing street quarters. Nonetheless, Churchill spent many working days and evenings during the war in the room and records show that he went to the map room daily.

The last major office is the war Cabinet room itself. The seats are marked with the names of such celebrated occupants as Clement Atlee, leader of the Labor Party, and Anthony Eden, Churchill's successor as Conservative prime minsister.

There are at least three layers of protective covering overhead. The first consists of timber beams that were placed in 1938. Later steel girders were added, and finally, once Germany began to bomb London, a three-foot concrete cover was constructed. There is no indication that the bunker was ever hit, although Buckingham Palace and the Parliament building, also nearby, were damaged.

In recent years, occasional small groups of visitors were shown through the rooms by special application, but the complex remained officially closed. About a year ago the decision was made to turn it into a full-fledged tourist attraction, and architects were hired to see how the unique and cramped character of the offices could best be displayed to an expected 300,000 visitors a year.

A.J.P. Taylor, a leading British historian and specialist in World War II, complained to the Sunday Times newspaper last weekend about the plans. He said the rooms were "the center of a great crusade. Nowhere else represents so well the way that the war was fought . . . It would be a great pity to vandalize it."

Environment Minister John Stanley replied that the offices are a national asset that should be available to the public. That access, he said, would be "reconciled" with the need to preserve the war rooms in their present remarkable atmosphere, a corner of London where time stopped in August 1945.