David Cameron and Ed Miliband will skip their weekly Prime Minister's Questions clash to travel to Scotland.
Both leaders, and Lib Dem Nick Clegg will head north to to tell the Scottish people: "We want you to stay."
In a joint statement the three said: "There is a lot that divides us - but there's one thing on which we agree passionately: the United Kingdom is better together."
But First Minister Alex Salmond said the visit was a sign of panic.
BBC political editor Nick Robinson said the collective gathering was "hugely significant".
In a further move, the Scottish flag, the Saltire, has been raised over Downing Street, with No 10 saying it will continue to fly until the referendum on 18 September.
Mr Miliband has also called for cities, towns and villages across the UK to display the flag.
Although the three leaders will all be in Scotland on Wednesday they will not travel together or appear together.
But in their statement they emphasised that keeping the UK together was now their priority.
"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," they said.
"We want to be listening and talking to voters about the huge choices they face. Our message to the Scottish people will be simple: 'We want you to stay.'"
Prime Minister's Questions is not being cancelled. William Hague will stand in for Mr Cameron, while Harriet Harman will deputise for Mr Miliband,
BBC Political Editor Nick Robinson's analysis:
The message in the leaders' joint statement is modelled on what was used by pro-Canadian campaigners in the Quebec referendum.
What happened in Quebec was the Yes side pulled ahead. There was a panic that Canada might be broken up.
One of the things that is said to have pulled that back in a very, very, narrow victory for the No campaign was the message coming from the rest of Canada saying: "We want you to stay."
Mr Cameron and Mr Miliband agreed on the move when they met in the Commons on Monday immediately after the Prime Minister's statement on the NATO Summit.
But Mr Salmond said: "The No campaign is in complete and utter disarray, and they are making this farce up as they go along.
"Together, David Cameron, Ed Miliband and Nick Clegg are the most distrusted Westminster politicians ever - and their collective presence in Scotland will be another massive boost for the Yes campaign.
"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."
A new poll suggests there is very little between the two sides in the Scottish independence referendum campaign.
Nick Robinson said the decision by the leaders to not take part in one of their constitutional duties but to head to Scotland instead showed "how seriously these Westminster politicians take the opinion polls".
"Nothing is more important to these Westminster politicians than to stop that Yes vote - all the normal political hostilities are on hold," he added.
'I'll do everything I can'
Mr Cameron said the three leaders hoped to make clear to the people of Scotland they could have "the best of both worlds" if they rejected independence - of being given more powers to govern themselves while also being inside the UK.
"I'll do everything that I can..." said Mr Cameron, explaining his decision to cancel his appearance in the Commons.
"We will all have our own ways, separately, of talking about why we are better together.
"But one thing I'm sure we will all say is that it's a matter for people in Scotland to decide, but we want you to stay."
Mr Miliband added: "There's no doubt and no uncertainty that greater devolution will happen in the event of a No vote."