By Adrian Masters Political reporter, BBC News

What's the story?

Opposition parties hope to defeat the assembly government over its spending plans and force changes to its planned budget. The maths of the assembly mean that opposition parties can defeat Labour but if the independent AM John Marek - the deputy presiding officer - was forced to chair the debate the same maths mean there'll be stalemate.

What's happening on Tuesday?

Assembly members debate and vote on the assembly government's draft budget. Finance Minister Sue Essex has published her proposals for spending the assembly's �14bn budget along with �74m of extra spending.

The opposition parties have tabled an amendment which amounts to a "shopping list" of spending they want to see included in the plans. If they vote together (including Dr John Marek) they can pass that amendment which has the effect of defeating the draft budget and forcing the assembly government into talks with the other parties.

Does this mean the assembly government's budget will be scuppered?

No. The assembly government's final budget will be voted on before the end of the year. These are still draft proposals and subject to negotiations.

Who usually chairs these debates?

Dafydd Elis-Thomas is the Assembly's presiding officer and chairs the majority of debates in the chamber. As his deputy, Dr Marek often fills in for him.

Why does it matter who chairs the debate today?

Assembly politics is finely balanced. Labour has 29 AMs while the combined opposition has 31 seats. Lord Elis-Thomas is usually out of the equation which leaves 30 opposition members against Labour's 29.

That means that when the opposition parties unite as they did over last year's budget they can defeat the assembly government.

It was finally confirmed on Tuesday lunchtime that Lord Elis-Thomas would chair the afternoon's plenary session - including the debate on the Budget.

If Dr Marek had been in the chair - and unable to vote - then the united opposition would have been only likely to muster 29 votes and would be unable to defeat the government - stalemate.

Why didn't it appear that Dafydd Elis Thomas wanted to chair this debate?

The official reason he had given was that today was a long day in the assembly and, as happens from time to time, he would like his deputy to chair some of the proceedings.