A key Scottish independence debate between Alex Salmond and Alistair Darling will not be broadcast to TV viewers in England, Wales and Northern Ireland.

The debate on Tuesday night, which comes ahead of the referendum on 15 September, will only be shown on TV by STV, the Scottish holder of the ITV franchise.

Anyone from England, Wales or Northern Ireland interested in the debate will have to tune in online and watch the programme, Salmond and Darling: the Debate, on the STV website.

Viewers outside of Scotland have expressed surprise and anger that no broadcaster is showing the programme, which airs between 8pm and 10pm, in the rest of the UK.

An ITV spokeswoman said: “ITV will screen the 8-10pm debate live, in the area where our viewers have a vote in the question under discussion. Viewers in Border Scotland will be able to watch the debate on Freeview. The debate will of course be covered as a news story in our news programmes broadcast to the whole of the UK.”

STV runs the channel 3 network in Scotland and therefore has a relationship with ITV. But ITV will tonight be showing Alan Titchmarsh-fronted show Love Your Garden and Kids Behind Bars, a documentary about American young offenders.

Similarly its other services ITV2, ITV3 and ITV4 also have no room for the debate, fronted by STV’s political editor Bernard Ponsonby from the Royal Conservatoire of Scotland in Glasgow.

STV’s website informs viewers how to watch the debate, which starts with the results of a poll on voting intentions carried out by Ipsos-MORI.

The website says the rest of the UK’s viewers will have to watch the debate online with a “globally available livestream” on the STV Player website. It adds that the usual requirement to enter a Scottish postcode will not be necessary.

The STV Player will also make available a full replay of the debate “within hours of the end of the programme”.

“STV expects there to be a very significant online audience watching the debate through the STV Player this evening,” said a spokeswoman for STV. “We are pleased to have secured the debate and believe this to be a landmark moment.”

Viewers in the rest of the UK may still get the opportunity to watch a debate between the pair.

It is understood the BBC is close to signing off another debate between the two, which would be expected to be made available to all UK viewers.

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