The future of England and Scotland's union is on a knife edge, the chairman of Parliament's Defence Select Committee will warn today, as he accuses the No campaign of a "failure of imagination" in their efforts to persuade Scottish voters to stay with Britain.

Rory Stewart said he was "very concerned" that Alex Salmond could win next week's independence referendum and said he believed there was "only 5 per cent in it either way". Mr Stewart, who represents Penrith and the Border, is the most senior Westminster figure to express publicly concerns held by many senior politicians that the Scottish referendum is slipping away from them.

Recent polls have shown the gap between the Yes and No votes narrowing sharply. The closeness has led to recriminations that David Cameron made a miscalculation when he refused to allow a "third option" of more powers for the Scottish Parliament to appear on the ballot paper.

Mr Stewart said he wished the No campaign had made more effort to promote the shared value of the union and "Britishness".

"Partly because I'm in a border constituency and because I'm a Scot, I have felt for a long time there was a decent probability of a vote for independence," he said.

"I just felt there was so much ferocity and energy in the campaign. The added problem has been that we have not had anybody making a real case for Britain. I have always felt the only way you can answer the argument in favour of independence is to say: Britain is an amazing country that's got an incredible future, and you wouldn't want to walk away from that." But he added: "We're on the coach – we've just got to let them play the strategy they've worked out. All I can do is keep my fingers crossed.

"I made a speech in Parliament – and it was barely reported in the press and the chamber was largely empty. It's very odd... If this had been happening 40 years ago or 100 years ago, it would have been the great issue of the day, and yet it hasn't felt like that at all. It's been curiously inert."

He said: "The failure of imagination has been not to say that these issues are party political and could be changed by a different government. They are not the things that should be resolved by a constitutional change. It's not an argument to rip the country apart."

He warned that a Yes vote would be devastating for all Westminster leaders. "I think he [Cameron] would feel it as a heart-breaking situation. It would be devastating for him personally and I think it would be an indictment upon all of us. I think all the party leaders would feel terribly, terribly guilty.