* Tennis fans to descend on Wimbledon from Sunday
* Thousands set to camp and line up for tickets
* Scattered showers forecast for start of championships
LONDON, June 21 (Reuters) - Tennis fans will descend on Wimbledon from Sunday to join Britain's line of the year in which thousands wait patiently to see their sporting idols during the two week championship.
Thousands will camp out overnight or arrive at dawn each day to join the line, known as The Queue, which has become as much a tradition at the world's oldest tennis tournament that dates back to 1877 as strawberries and cream - and rain.
For Wimbledon is one of the few major British sporting events where people can still buy premium tickets on the day, if they are prepared to spend up to 24 hours in a line.
But newcomers beware because The Queue has transformed over the years to epitomise the British obsession for orderly lines, with strict rules ensuring fairness and civility and a list of websites and Twitter accounts giving tips on how to queue.
"QUEUE JUMPING IS NOT ACCEPTABLE AND WILL NOT BE TOLERATED," shouts a message on the Wimbledon website.
As people join the line they are handed a card that is dated and numbered and has to be given in at the ticket office where several thousand tickets are sold daily at 20 pounds ($31) each for unreserved seating and standing room on courts 3-19.
Wristbands are issued only to those at the front of the line, who then pay from 45 pounds ($70) for one of the 1,500 tickets available each day for the top three courts - Centre, One, and Two.
The rules, set by Wimbledon's organisers, the All England Lawn Tennis Club (AELTC), state no saving places for others, no reserving spots with equipment then nipping off somewhere more comfortable, and no loud music in the line.
A Wimbledon spokesman said the rules were introduced about 10 years ago to ensure fairness and were refined over the years - and the line has grown ever since.
QUINTESSENTIAL ART OF LINING UP
Veterans of The Queue enjoy their annual wait almost as much as the tournament, relishing the quintessential British art of queuing that etiquette experts say dates back to the world wars last century when Britons had to line up for food rations.
For six years Dex Hill, 18, a student at London's Kingston University, has been among the first to set up his tent in the line that starts at 8 a.m. on Sunday, and he will be there this year, hoping to get a good ticket for the Monday start.
"Being in the queue is really good fun because everyone is there for the same reason, talks tennis, and there is almost a party atmosphere," Hill told Reuters.
"Tips? Get there early and be prepared for any weather."
Jo Bryant, etiquette advisor from Britain's authority on manners, Debrett's, said the art of queuing must seem esoteric at best and maddening at worst to foreigners.
"But queue-barging is the worst solecism a foreigner can commit. Even the reticent English will feel justified in sharply pointing out the back of the line to any errant queue-jumpers," Bryant told Reuters.
The fans in line, however, do have to cope with Britain's fickle summer weather and the standing joke in Britain is that tennis fortnight means rain.
Meteorologists from Britain's national weather service, the Met Office, predict scattered showers for Wimbledon next week.
Rain stopping play prompted the AELTC to install a retractable roof on its main Centre Court four years ago and the club this year announced plans to put a roof on Court 1 by 2019.
Whatever the forecast, up to 38,500 tennis fans are expected daily at the All England Club in Wimbledon, south London, as the event is as much social as sporting with corporate tents galore.
Over the two weeks 8,615 punnets of strawberries and 200,000 glasses of the traditional cocktail Pimm's will be sold as players battle for the record 1.6 million pounds top prize money.
"There really is nothing quite like Wimbledon," Hill said. ($1 = 0.6466 British pounds) (Editing by Peter Rutherford)