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Welsh politicians and commentators last night hit back at the Prime Minister’s offer of unspecified extra powers to Scotland should voters there reject independence.

Plaid Cymru said the Edinburgh visit was a “PR exercise” and Professor Richard Wyn Jones, a leading political scientist at Cardiff University’s Wales Governance Centre, warned Mr Cameron was in danger of looking “shifty”.

Prof Jones said: “From what I’ve gleaned he hasn’t been able to provide any detail at all about what more powers might mean. He’s in danger of finding himself in a position where he’s looking pretty shifty because he’s offering more [power] without being able to specify what that might be.”

Read more: David Cameron hails the UK as a "warm and stable home billions envy"

Prof Jones also acknowledged the lack of an obvious leader for the pro-union campaign to counter Mr Salmond’s message.

He added: “Because of the scale of the SNP’s victory all the other parties have replaced their leaders.

“None is particularly high-profile, experienced or charismatic.

“The UK leaders just can’t go there because it sends all the wrong signals. Everywhere they look, it’s very, very difficult.”

Prof Jones also noted that the debate over the future of the UK has gripped the imagination of Welsh politicians.

He said: “If you talk to anybody active in Welsh politics in any party this is something they are almost transfixed by. It would be a mistake to think this is simply a concern of Plaid activists.”

Read more: Alex Salmond criticises David Cameron's offer of more powers to the Scottish Government.

Former Welsh Office minister and ex-Conservative Assembly leader Rod Richards warned that in making the financial case for staying in the union the prime minister must not suggest that Scottish voters are the recipients of a “hand-out”.

He said: “He needs to be careful. Unless he’s very, very careful how he puts it, it creates the impression Scotland is getting a hand-out from England.

“It’s not a hand-out. We are one country and therefore transfers of money from the Treasury around the country are a perfectly normal thing to do.”

Constitutional expert Alan Trench said he doubted that Mr Cameron’s description of the role Scotland can play on the world stage that as part of the UK would have wide appeal.

“Cameron is treading a difficult line,” he said. “He has to be seen to be engaged but I think he’s well aware as an electoral asset to the pro-union position of limited value...

“He’s talked about international organisations and the UN and they are unlikely to appeal to you average voter.”

A Plaid Cymru spokesman said: “The independence referendum in Scotland is a contest between those who want a better future and the forces of Conservatism. Mr Cameron is the figure-head for those who will try to gloss-over the failings of the union, and the legacy of economic failure it has created.

“As in Wales, if the people of Scotland want to create a more prosperous future for themselves, they will have to take responsibility and control of their own fortunes. No Conservative PR exercise will change that fact.”

A Welsh Labour spokesman described the debate as a “sideshow”.

He said: “As recent polls show, independence is a sideshow in Wales. Even a majority of Plaid supporters are in favour of a United Kingdom.

“That is why this coming weekend, Welsh Labour’s annual conference in Cardiff will be focused entirely on the real priorities facing the people of Wales – jobs, the economy, health and education.”