Opinions are split on the extent of the assembly's powers

Mr Jones, who heads the party in the assembly, wants the issue discussed once the Richard Commission has published its report in March.

In a speech in Cardiff on Monday he will warn against a Labour "stitch-up", while setting out his party's arguments for a Welsh parliament.

Earlier this week, First Minister Rhodri Morgan confirmed that he favoured devolving full law-making powers to the assembly.

But he drew the line at allowing it to raise taxes.

Mr Jones, however, will outline his arguments for more radical change.

He wants a review of the Barnett formula - the system which calculates how much money Wales gets from Westminster - and he wants the assembly to be able to vary Income Tax and Corporation Tax.

Ieuan Wyn Jones : demanding action

Plaid hopes to draw attention to its campaign for a Welsh parliament with a rally in Machynlleth later this month.

The event will be held on 28 February - the 600th anniversary of Welsh hero Owain Glyndwr establishing his parliament in the town.

The assembly's powers - or lack of them - were thrown into the spotlight on Tuesday, when Rhodri Morgan spoke to assembly members during first minister's question time.

His comments gave the clearest indication so far on two of the big issues for the future of devolution.

He said: "The broad thrust of my own personal belief is that it would be better if we had primary legislative powers and it would give us more capability to do more good things for the people of Wales, but I take the completely opposite view on tax-varying powers."

His comments came as the Richard Commission - the body looking into the future of the devolution settlement in Wales - put the finishing touches to its final report.

Under wraps

Both Plaid Cymru and the Liberal Democrats welcomed his words.

Mr Morgan told AMs that he had chosen to keep his views largely under wraps while the commission was drawing up its report.

The commission is due to publish its report on 31 March.

The UK Government is understood to have declared three "no-go" areas on the issue of further powers for the assembly.

These are thought to be tax-varying powers, any cut in the number of MPs, and anything requiring a referendum.

The commission, set up in 2002, has been examining the assembly's work since the devolved body was set up in 1999.