Britain's three main party leaders will cast aside partisan Westminster politics on Wednesday, abandoning the routine of the weekly battle of prime minister's questions, to travel to Scotland in a desperate joint bid to stop a haemorrhage of votes towards Scottish independence.

Travelling separately and speaking to different audiences, David Cameron, Ed Miliband and Nick Clegg will fan out across Scotland on Wednesday to make an impassioned collective appeal to Scots to vote to stay inside the what they will say be a changedUnited Kingdom.

"There is a lot that divides us – but there's one thing on which we agree passionately: the United Kingdom is better together," the leaders said in a joint statement on Tuesday. "That's why all of us are agreed the right place for us to be tomorrow is in Scotland, not at prime minister's questions in Westminster.

"We want to be listening and talking to voters about the huge choice they face. Our message to the Scottish people will be simple: 'We want you to stay.'"

Alex Salmond, the first minister of Scotland, described the trio's decision to travel north as "the biggest blunder of the campaign" and claimed the Westminster establishment was "in a total and utter panic" as its campaign disintegrated.

The unprecedented cooperation across the Westminster divide exposes the naked fear in London that Scottish voters are not heeding the jitters in the financial markets or dark warnings about the irreversible risks to the Scottish economy entailed in a vote for yes next week.

The three leaders arrive a day after Mark Carney, the governor of the Bank of England, warned that a currency union was "incompatible with sovereignty", rejecting Salmond's claim that Scotland could share the pound and become an independent nation. The governor set out three elements necessary for a successful currency union: free movement of capital, banking union, and joint fiscal arrangements over tax and spending.

The currency debate is being watched closely by financial markets, along with the approach that would be taken by the banking industry in the event of a yes vote. Analysts at Credit Suisse said Scotland would fall into a deep recession in the event of a yes vote. "We think deposit flight is both highly likely and highly problematic," they said, pointing out that Scotland's bank assets were 12 times GDP.

The Queen also stepped into the debate, with Buckingham Palace releasing a statement saying the outcome of the independence referendum is "a matter for the people of Scotland". Currently in Balmoral, where she will stay until after the vote on 18 September, her intervention came after Salmond suggested she would be "proud to be Queen of Scots" and the Sunday Times said she had a "great deal of concern" about a yes vote.

Cameron had not been expected to travel to Scotland until the final week of the campaign, but the prime minister agreed to tear up this timetable after a meeting with Ed Miliband on Monday afternoon at which the two men discussed a second opinion poll from TNS that had the yes vote surging to 38% and in effect tied with the no camp on 39% once undecided voters are excluded.

Cameron ordered the saltire to fly over Downing Street until the referendum is concluded, as Miliband urged towns and cities across the UK to fly the flag in support of the union.

Miliband's team rejected the prospect of the three party leaders appearing together on a joint platform, fearing the Labour leader would be tainted if he appeared alongside Cameron, and might come across as part of a bullying Westminster elite entering Scotland to demand surrender terms. "We agreed to march separately but to strike together," said a Labour source.

Salmond was scathing about the politicians' arrival in Scotland, pointing to opinion polls giving all three leaders a substantial negative rating – adding up to minus 150 points in the latest YouGov poll. That made them the most distrusted Westminster politicians ever, Salmond said. He added: "The message of this extraordinary, last-minute reaction is that the Westminster elite are in a state of absolute panic as the ground in Scotland shifts under their feet." He even offered to pay for their travel costs.

In what appears to be a deliberate attempt to offer rolling news networks a stream of pro-UK events, Cameron is due to appear in the Edinburgh area in the morning, followed by an event featuring Clegg in the Scottish Borders, while Miliband is due in the Glasgow area later.

In his speech in Glasgow, Miliband will deliver the strong message of change that the former prime minister Gordon Brown is now sending to Labour supporters in an attempt to reassure them the choice next week is no longer between an unsatisfactory status quo or separation.

Reminding voters that they would be casting their vote in indelible ink, Cameron writes in the Daily Mail that he "agrees wholeheartedly" with Brown's timetable for further devolution. He says: "This is the sort of clarityyou need as you make this decision, especially when you're not just making this decision for yourself, but for your children, your grandchildren, and their children too.

"With this timetable, we are giving people that clarity, showing that by voting no, Scotland gets the best of both worlds: power over the policies that matter, and the stability of the United Kingdom; the freedom to chart its own destiny, and the support of three other nations; the reputation in the world as a successful nation, and the clout of a world-renowned union".

He adds: "While a yes vote may be a lucky dip, a no vote is a guaranteed win for anyone who wants a stronger, more autonomous Scotland."

Miliband will say on Wednesday: "A vote for no is not a vote for no change, it is a vote for change, a vote for change in terms of more devolution of power, and a vote for change in the way our economy and our country works, because we've heard the call for change from the voters of Scotland."

Some Labour sources acknowledged that alongside an emotional appeal for the UK to stay together the three leaders needed to put pressure back on Salmond to explain how Scotland could operate alone. "It has got to the position where he appears like a commentator on the campaign," said one frustrated London official.

The last party political broadcast from the cross-party Better Together campaign is aimed at wavering and rebellious Labour voters after the latest polls suggested up to a third of them intend to vote yes in the referendum.

It features emotive black-and-white-footage of the Jarrow jobs march, of the early NHS and promotes Labour's role in introducing the minimum wage and the Scottish parliament. It ends with Brown speaking directly to the camera, where he says: "I love Scotland. It's as simple as that. I'm proud of our history and of our culture."