If today’s carnage in Madrid turns out to be the work of Eta, it would be the worst attack by the Basque separatist terror organisation.

The group - Western Europe’s most active guerrilla force - has been responsible for the deaths of almost 850 people since its campaign of terror began in 1968.

Eta, which stands for Euskadi Ta Askatasuma, meaning "Basque Fatherland and Liberty" in the Basque language, was formed in 1959 by young activists in the wake of General Franco’s moves to ban the Basque language and clamp down on the distinctive culture of the area.

Since then it has been fighting for the independence of the Basque country in the far north of Spain and southwestern France.

Basques are a linguistically and culturally distinct Christian group that has lived since the Stone Age in the mountainous region that straddles the border between modern-day Spain and France.

The Basques have never had their own independent state, but they have enjoyed varying degrees of autonomy over the centuries under Spanish and French rule.

About half of the 2.1 million residents of the three provinces that make up the autonomous Basque region speak fluent Basque or understand some of the language.

Eta’s bloodshed hit its peak in the 1970s and 1980s with scores of car bombs and gun attacks against judges, politicians and police, as well as a concerted targeting of the Spanish tourism industry.

In recent years it has also targeted journalists and civilians.

Eta has been under pressure in recent years with 179 suspected members detained during 2003 and a total of 650 suspects arrested since 2000, mostly in Spain and France. As a result, only three people died in 2003 compared with 23 in 2000.

One of the worst atrocities committed by Eta before today was a bomb in a Barcelona supermarket on June 19, 1987 which left 21 people dead and 45 injured.

In 2002 the Spanish Government banned the Basque political party Batasuna, which it described as ETA’s political wing. The party denied the charge. Batasuna won 10 percent of the vote in Basque parliamentary elections in May 2001.

There is no credible evidence that Eta has links to al-Qaeda, however al-Qaeda cells have been discovered in Spain.

Last month, Eta declared a ceasefire limited to the northeastern region of Catalonia but made clear it would pursue the armed struggle in the rest of the country.

Less than two weeks ago police arrested two suspected Eta members who were heading for Madrid with a van containing 500 kg (1,100 lbs) of explosives, averting a possible massacre in the capital.

José Maria Aznar, the Spanish Prime Minister, who survived an Eta attack while he was opposition leader in 1995, made eliminating the group a top priority.