Britain will be a more "insular and inward-looking country" if it votes to leave the European Union, David Cameron said as the marathon referendum campaign entered its final 24 hours.
The Prime Minister hit out at the Leave campaign's "narrow" focus on immigration and heavily criticised Ukip leader Nigel Farage's "breaking point" poster depicting non-white migrants in mainland Europe as an attempt to breed intolerance.
In an apparently co-ordinated response from the Remain camp to Leave's perceived advantage on the issue of immigration, London Mayor Sadiq Khan used a showpiece BBC TV debate to accuse Boris Johnson of peddling "project hate" against migrants.
But Mr Johnson received a standing ovation from parts of the 6,000-strong Wembley Arena audience when he declared that polling day on Thursday "could be our country's independence day".
Mr Cameron, who will hit the campaign trail for a final time to squeeze out every last Remain vote with the outcome of the referendum on a knife-edge, said immigration needs to be handled with care because Britain is "arguably the most successful multi-ethnic, multi-faith, opportunity democracy anywhere on earth".
The PM told the Guardian: "I've always believed that we have to be able to discuss and to debate immigration. But I've always believed that this is an issue that needs careful handling.
"We are talking to a country that has a lot of people who have fled persecution and contribute a massive amount to our country. It does need great care."
Leaving the EU would send "a very clear message that we've rejected this idea, that Britain is narrow and insular and inward-looking", he said.
But with bookmakers convinced Remain has the upper hand and markets rallying with the pound touching its highest point since the start of the year as investors bet on Britain staying in the EU, Mr Cameron identified Mr Farage's poster as a "bit of a moment" when the campaign may have turned.
"I think people looked at that and just thought, 'eurgh'.
"The motivation was just so obvious, that it was an attempt to divide and breed intolerance for political benefit, and it was the wrong approach."
Meanwhile, Mr Cameron predicted an investment boom if Britain backs staying in the EU, although he admitted the referendum was "very close" and that "nobody knows what's going to happen".
He told the Financial Times: "I think on Friday that businesses, wealth creators, job creators will think: Britain has made a decision, let's pile back into the economy and create jobs and opportunity because it's a great place to do business."
Mr Cameron also told the paper he was "frustrated" more organisations have not "put their heads over the parapet" to back Remain, but will be cheered by a pro-EU letter to the Times signed by 1,285 businesspeople employing 1.75 million people.
The signatories include 51 FTSE 100 companies and 910 small businesses, and was backed by the likes of Sir Richard Branson, Carphone Warehouse boss Sir Charles Dunstone and Ann Summers chief executive Jacqueline Gold.
During Tuesday's TV debate Mr Johnson, who will also be out campaigning on the final day, criticised Remain for "woefully underestimating" the country.
He said: "At the end of this campaign I think you'll agree there is a very clear choice between those on their side who speak of nothing but fear of the consequences of leaving the EU, and we on our side, who offer hope.
"Between those who have been endlessly rubbishing our country and running it down, and those of us who believe in Britain.
"They say we can't do it, we say we can. They say we have no choice but to bow down to Brussels, we say they are woefully underestimating this country and what it can do."
He added: "If we vote Leave and take back control, I believe that this Thursday can be our country's independence day."
Jeremy Corbyn will lead Labour's final charge for a Remain vote by again insisting the EU needs reform, but that staying in will protect jobs, workers' rights and the NHS.
Ahead of a rally in London with Labour leaders from across the UK, he said: "Throughout our history, the people of Britain have looked outwards, and for ways to work with others to improve the lives of our fellow citizens and the world around us.
"It is this commitment to delivering real change for all which forged the Labour Party.
"We believe Britain is better off in the European Union, not because we don't think Europe needs reform – it clearly does – but because it offers the best cross-border framework that we have to defend living standards, rights and protections for all our people.
"By voting to remain we can protect jobs linked to Europe, defend workers' rights from Tory leaders who want to scrap them, and safeguard our NHS from the threat of runaway Tory cuts and privatisation.
"So I urge Labour supporters this Thursday: do what's best for our people. Vote for jobs, rights at work, and our NHS. Vote Remain."