The expulsions came amid reports of a move towards a ceasefire leading to a dialogue with the government in Madrid.
According to an internal journal, quoted by a regional newspaper on Monday, the six were expelled from Euzkadi Ta Askatasuna ranks for refusing to withdraw comments made in a letter published in August 2004.
"The armed struggle that we develop today is good for nothing," said the prisoners, including former Eta leaders Francisco "Pakito" Mugica Garmendia, Inkaki "Makario" Arakama Mendia, "Inaki de Lemona" Bilbao Beaskoetxea and Karlos Almorza Arrieta, alias "Pedrito de Andoain".
Switch in strategy
The six called for a switch in strategy to "mass struggle and institutional struggle", saying "our political-military strategy has been superseded by the repression of the enemy".
The Eta leadership accused the prisoners, who are detained at Puerto II prison in southern Cadiz, of "a clear lack of discipline" and of trying to "create opinions contrary to the sense and the line of action" of the organisation.
Their behaviour "has given the enemy the opportunity to make publicity" and indicated "a lack of confidence, contempt and lack of respect for the organisation", Eta said in a bulletin this month.
"The armed struggle that we develop today is good for nothing"
Imprisoned former Eta leaders
The expulsions were condemned by the non-violent leftist Aralar party, which said "the discipline of an organisation cannot take precedence over the [peace] demands of the immense majority".
The Basque pacifist group Elkarri said the action was "worrying" as "what the prisoners said is what most of society thinks".
However, sources from the banned Batasuna party, Eta's political wing, told El Pais newspaper that the peace process was "irreversible".
Give up the fight
The fact that Eta, which this year claimed responsibility for a series of minor blasts, had not carried out a fatal attack since mid-2003 indicated that it was ready to give up the armed fight, El Pais said.
Eta is blamed for more than 800 deaths in a 40-year campaign to win an independent Basque homeland straddling the Pyrenees in northern Spain and southwestern France.
Spain's socialist government has said it will talk to the outlawed organisation if it disarms. A number of Eta members have been arrested in recent months by Spanish and French police.