David Cameron raised eyebrows and attracted headlines with one of his biggest intervention yet in the Scottish independence debate this lunchtime.

The prime minister has repeatedly indicated in the past he feels his 'Tory toff' reputation would only hinder the efforts of the unionist Better Together campaign, which is headed by former chancellor Alistair Darling.

But today Cameron delivered a heartfelt speech from the Olympic park in east London in which he encouraged the entire country to persuade Scottish voters to choose to stick with Britain.

"There are 63 million of us who could wake up on September 19th in a different country, with a different future ahead of it," he said.

"That's why this speech is addressed not to the people of Scotland, but to the people of England, Wales and Northern Ireland.

"We would be deeply diminished without Scotland. This matters to all our futures. And everyone in the UK can have a voice in this debate.

"If we lost Scotland, if the UK changed, we would rip the rug from under our own reputation. The plain fact is we matter more in the world together."

Cameron faces criticism from the SNP, which is struggling in the polls and hopes to change that by persuading the PM to debate the relative merits of Scottish nationalism.

Nicola Sturgeon said the PM delivering "high-handed lectures from the safety of London" would not help win over Scottish voters.

"He has to come to Scotland and go head-to-head in good old democratic fashion with the first minister. For some reason he doesn't want to do that," she complained on the Today programme.

"He doesn't have the courage of his convictions, he doesn't have the guts to test his argument against the arguments of Alex Salmond.

"Today I think should be a good sign for the yes campaign because it signals the jitters at the heart of the no campaign. 'Project Fear' doesn't seem to be working."

There are now just seven months to go until the independence referendum on September 18th, when four million Scottish voters will choose whether to end 300 years of union with the rest of the UK.

Cameron's speech is likely to resonate with English and Welsh voters. A YouGov poll released earlier this week found 54% are hoping for a 'no' vote – a shift from polling three years ago, when a majority of English and Welsh people preferred the Scottish to leave.

"To everyone in England, Wales and Northern Ireland – everyone, like me, who cares about the United Kingdom – I want to say this: you don't have a vote, but you do have a voice," Cameron added.

"Those voting are our friends, neighbours and family.

"You do have an influence. Let the message ring out, from Manchester to Motherwell, from Pembrokeshire to Perth, from Belfast to Bute, from us to the people of Scotland – let the message be this: We want you to stay."

The prime minister's decision to deliver his speech at the Olympic park is being viewed as a deliberate attempt to politicise the legacy of the Olympics on the same day as the Sochi 2014 opening ceremony in Russia.

"For me, the best thing about the Olympics wasn't the winning. It was the red, the white, the blue," Cameron said.

"It was the summer that patriotism came out of the shadows and into the sun, everyone cheering as one for Team GB."

Memories of Alex Salmond unveiling the Scottish saltire after Andy Murray won Wimbledon last year are undermining nationalist complaints about the move, however.