MORE than 1.7 million viewers watched the historic referendum debate on television and on the STV player as it smashed the channel's viewing figures for political broadcasts.

Alex Salmond and Alistair Darling's head-to-head on Tuesday night was watched by an estimated 1.2m viewers and another 500,000 live online in spite of the system creaking under the levels of demand.

The television audience for the event, hosted by Bernard Ponsonby at the Scottish Conservatoire in Glasgow, achieved a 36 per cent share of viewing, making it the best performing political debate in Scotland for over a decade.

The peak audience at any one time was 920,000 and the average across the whole programme was 765,000. To put that in context, the main televised debate ahead of the last Scottish Election achieved an average of just under 700,000.

Bobby Hain, director of channels at STV, said: "This landmark TV debate delivered ratings that make it the best watched political debate in Scotland for more than 10 years.

"We are aware that, although the live online stream achieved over half a million viewers, due to extremely high demand some viewers had difficulties accessing the programme.

"We apologise to anyone unable to watch the debate in full. The programme is available on our catch-up service, STV Player."

August 25 has been pencilled into many political diaries as the date of the likely BBC version of the independence referendum debate between Mr Salmond and Mr Darling, but this has not yet been confirmed.

There have been arguments over the last month about the preferred date of the BBC debate, with both sides clashing over how late in the campaign it should come in the light of the date for postal votes.

Both sides of the debate indicate that the details for an August 25 clash are now close to being agreed. Channel Four and Sky News are keen to screen clashes between the camps UK-wide.

Meanwhile, Prime Minister David Cameron, on holiday with his family in Portugal, has yet to see all of the debate. A No 10 source said he had been "fully briefed" on the outcome.

There were complaints south of the border that a debate potentially impacting on the rest of the UK was not broadcast across the ITV network.