DAVID Cameron has welcomed Alex Salmond's decision to engage in a TV debate with Alistair Darling on the independence question, insisting he has always felt the referendum debate should be led by Scots.

The First Minister "blinked", say his critics, when he announced at the weekend he would be willing to debate Mr Darling in the absence of the Prime Minister, who has steadfastly refused to accept Mr Salmond's challenge. Pundits suggest he would have more to lose in such a showdown.

Nonetheless, Mr Salmond has not abandoned his call for Mr Cameron to engage him in a live debate. Yesterday, the SNP pointed out how Mr Salmond had agreed to a proposal from STV to participate in a live debate before a studio audience in Edinburgh on July 16 if the PM accepted too.

Annabelle Ewing, the Nationalist MSP, urged the Tory leader to "step up to the plate", saying: "David Cameron should have the courage of his convictions and agree to the July 16 debate with the First Minister but, if he continues to duck it, the First Minister will take on Alistair Darling shortly after the Commonwealth Games."

Given No 10's opposition to a PM/FM debate with one source saying it would "never" happen, Coalition ministers became increasingly sure Mr Salmond would debate with Better Together leader Mr Darling.

Earlier this month, Gordon Brown, the former Labour Prime Minister, caused eyebrows to be raised when he suggested it would be a "good idea" if the two leaders went head to head. Some of his colleagues in the No camp were not best pleased.

Asked Mr Cameron's response to the FM's decision on accepting a debate with Mr Darling, his spokeswoman said: "We welcome the fact that the debate is going ahead. The Prime Minister wrote to Alex Salmond in September last year when he said the referendum TV debates should be led by Scots living in Scotland."

Ahead of a major debate on the constitutional future of Scotland in the House of Lords today, calls were made for a UK-wide "holistic" convention to consider all the constitutional issues.

Lord Foulkes, the Labour peer, said there was "growing concern at the piecemeal nature of constitutional reform" and asked: "Would it not be sensible for the Government now to announce that a constitutional commission is going to be set up to look at constitutional change throughout the whole of the UK in a coherent way and preferably before September 18?" he asked.

Lord Wallace of Saltaire for the Coalition said the UK Government had moved on further powers. He added: "The question is how we move; whether we go toward a UK-wide commission or indeed a convention...is one which we all need to consider."

l Harry Potter actor Daniel Radcliffe has said he wants Scotland to remain in the UK. Speaking in New York, Radcliffe said: "I personally like the UK being how it is."