Catalan nationalists look set to take half the vote in parliamentary elections billed as a proxy independence referendum.

A new poll suggests a recently forged alliance of "independentistas" will win an absolute majority of seats in the Iberian nation's parliament next week.

Crucially, the latest survey also signals that two pro-secession slates, the broad left-centre grouping Junts pel Sí and the radical left CUP, will collect 49.9 per cent of votes.

At a glance: the forces fighting Catalunya's proxy indyref

That puts them within touching distance of the absolute majority of votes that outside commentators believe would give them the democratic legitimacy to pursue a much-mooted "roadmap" to independence without a Scottish-style referendum.

That would put Catalunya's government on a collision course with a Conservative Spanish government determined to block any attempt at independence, including a democratic vote.

Parliamentary elections are scheduled for September 27 - or 27S for short. Only Junts pel Sí - which brings together pro-independence parties of the broad left and centre as well as grassroots activists - and the radical CUP are regarding this as a proxy referendum.

Many of their opponents see the vote as a normal parliamentary election and accuse independence supporters of hijacking the event.

The latest poll, published in Spanish newspaper El Confidencial on Friday, puts Junts pel Sí on 42.4 per cent of the vote with between 63 and 65 seats in the 135-member Catalan Assembly.

That is fewer seats than currently held by the two main constituent parts of the alliance, SNP allies Esquerra Republicana and the liberal centralist Convergència Democràtica de Catalunya.

This means Junts pel Sí will have to rely on the radical leftist of CUP to hold power and claim legitimacy. CUP, the El Confidencial poll says, will get 7.5 per cent of the vote and 10 or 11 seats.

The poll also suggests another huge blow for the Catalan Socialist Party or PSC, part of Spain's federated socialist party.

Until 2011 the Socialists always came first in the Catalan portion of Spain-wide elections.

The PSC, arguably the equivalent of Scottish Labour, looks set to get about as many seats as CUP.

Socialists have been squeezed by a triangle of different forces making them look increasingly irrelevant.

One on side is Junts pel Sí and CUP, mopping up many radical voters who no longer believe federalism is a realistic prospect.

On the second side is a loose alliance of green, former Communists, eco-socialists and the Catalan wing of anti-establishment and anti-austerity party, Podemos.

This united slate will take 10 per cent of the vote.

But the Socialists are also facing the self-styled "anti-nationalists" of Ciutadans or Citizens.

In what would be an historic breakthrough, this still relatively new party are now polling second highest in Catalunya.

The party representing the ruling Conservatives in Spain, the PPC, is also pulling clear in to third place, suggesting hardening unionist sentiment in Catalunya.

The El Confidencial poll is based on more than 1000 people and was conducted after a mass demo last weekend saw perhaps 1.5m Catalans take to the streets of their capital, Barcelona, to demand independence.

A poll taken just before the demo, which marks Catalunya's La Diada national day, showed nationalists short of an absolutely majority in both seats and votes.

The Catalan vote will be followed by Spanish-wide "national" elections later this year.