LONDON -- If you've never tried the echo in the hushed reading room of the British Library, you'd better hurry.

Time is running out to heed humorist Gerard Hoffnung's tongue-in-cheek advice to tourists and commit one of the worst sacrileges imaginable in Britain.

The countdown is on for the library and its legendary centerpiece, the domed reading room, to be removed from the British Museum in London's Bloomsbury to a new building in St. Pancras, half a mile north.

The new premises, the first part of which is to be completed in 1993 at a cost of $378 million, will be spacious, air-conditioned and equipped with the latest in computer-based library technology -- but its design is unlikely to tempt anyone to try the echo.

The library says the move is inevitable.

"It will provide a unique opportunity for preserving, managing, exploiting and presenting the national collections more effectively for the benefit of future users," it said in its "Strategic Plan 1985-1990."

But many of those who have worked in the old building will be sad to see the end of an era.

"When we move to the new building, an awful lot of history will disappear," said Phil Harris, who after long service as a senior employee in the library has started chronicling the first 220 years of its existence.

History has certainly been prepared -- if not actually made -- in the 33 rows of 375 leather-clad seats leading outward like the spokes of a wheel from the superintendent's desk and the catalogue in the center.

Somewhere in Rows K to P, a man who called himself Jacob Richter -- better known later as Lenin -- used to read the books of an earlier regular who still looms large in the location's folklore: Karl Marx.

"Ah yes, Mr. Marx, chap with a beard. I remember him well, he kept reserving books on economics. Did he ever amount to anything?" one staff member is reported as saying in response to an inquiry about the founder of communism.

Lenin and Marx, who profited from the library's unique collection for his theoretical study "Das Kapital," were among many prominent users including poet W.B. Yeats, novelists Sir Walter Scott, Charles Dickens and Thomas Hardy, dramatist George Bernard Shaw and naturalist Charles Darwin.

But the reading room has attracted not only the famous.

"I believe there are several people in a state of imbecility who come to read in the British Museum," reflected historian Thomas Carlyle on the rather mixed clientele.

They are what Harris loves about the place.

"It was always like Picadilly Circus. Grand as the new building will be -- it will be a long time before it acquires the atmosphere and the patina of the old," Harris said.

Library spokesman Ken Shirreffs said, "You could say it's a question of service versus esthetics."

The British Museum, which has been letting the Reading Room to the library for a token rent, has yet to decide what to do with it once its lessee moves.

The reading room is a remarkable example of Victorian cast iron architecture. The 106-foot-high dome filling an interior quadrangle flanked by the wings of the museum was built in 1857 by an exiled Italian revolutionary, Antonio Panizzi.

A man of relentless vigor, he sketched the first plan, battled to extract the necessary funds from the government and supervised every detail of construction, down to the design of desks, bookrests, metal struts and the leather covering the shelves to protect the books from rubbing.

Panizzi also enforced a law obliging publishers to deposit with the library one copy of every book or periodical printed.

The collections rapidly became "the memory of the nation."

In 1973, what until then was the British Museum Library merged with other bodies to form the British Library, the country's central reference and information facility.

As a permament archive for the sciences, arts, business, technology and industry, as a national museum of the book, preserver of Britain's literary heritage and a patent information network, the library claims a range of services unrivaled by any other in the world.

Resources stop just short of the impossible -- as one man discovered when he asked to see the autograph of Jesus Christ.

It houses some 16.5 million volumes of books, manuscripts, maps, newspapers, pamphlets, stamps, photographs, patents and music, half a million discs and 36,000 hours of recorded tape.

Treasures include the Magna Carta, a copy of Shakespeare's first Folio and a Gutenberg Bible.

The annual intake of 600,000 items requires nearly two miles of additional shelving in the book department alone.

Half the collections have had to be stored in Woolwich, east of London, and readers have to wait up to three days for a book to be brought to their seat in the reading room.