By Jane Barrett

MADRID, June 27 (Reuters) After four decades of separatist violence, Spain's prime minister will lay out plans for peace talks with Basque guerrillas ETA this week but fierce resistance from the opposition will make negotiations tougher.

Other Spanish governments have tried to end the bloody fight for Basque independence but Jose Luis Rodriguez Zapatero has the best chance in years after ETA, weakened by revulsion against global terrorism, declared a permanent ceasefire in March.

Then, the conservative opposition Popular Party (PP) gave Zapatero rare support to push ahead with the peace process but it pulled out in June when the Basque wing of the Socialists said it would speak to Batasuna, seen as ETA's political wing.

''When you talk to somebody who has a pistol as a negotiating tool, you have lost. It is illegal, immoral and useless,'' party leader Mariano Rajoy said at the weekend.

Analysts expect Zapatero to forge ahead without the opposition and formally announce talks in parliament. But opposition criticism could make it harder for him to cut deals and some analysts say ETA will worry that the conservatives might come back to power and reverse peace accords.

''(The lack of PP support) would be a major complication because the other side always wants guarantees that there's a basic deal between the main parties who might rule the country,'' said Julian Santamaria, political science professor at Madrid's Complutense University.

The PP reportedly talked to Batasuna and ETA when in power but after former truces broke it took a hard line against them.

Years of police operations weakened ETA, and Islamist train bombings in Madrid in 2004 made violence less acceptable, analysts say.

Under Zapatero, police have kept cracking down, arresting founding ETA members last week. But Spaniards have fumed against decisions not to jail Batasuna leaders and were revolted by the remorseless attitude of an ETA suspect in court last week for shooting a PP councillor in the head in 1997.

Nonetheless, a poll by Instituto Opina yesterday showed 60 per cent of Spaniards backed Zapatero's plans to talk to ETA and 55 per cent felt the opposition was wrong to pull its support.

Analysts expect Zapatero to focus on issues like disarmament and moving ETA prisoners nearer the Basque Country.

Talks between parties, possibly including Batasuna, on the future of the Basque Country would run separately. Most Basques do not want independence, polls say.

''We must get going as soon as possible because the road is going to be long and difficult,'' said Patxi Lopez, secretary general of the Basque Socialist party. ''I hope that even if we start without the PP's support they will end up joining in.'' REUTERS SHB HT1442