Politicians are making their final pitch for votes on the last day of campaigning before the EU referendum.
David Cameron, Jeremy Corbyn and Tim Farron are addressing rallies arguing the UK will be better off and safer with a Remain vote in Thursday's poll.
Boris Johnson and Nigel Farage are appealing to their own Leave supporters - with the ex-London mayor urging people to "believe in our country".
More than 46 million people are eligible to vote in the referendum.
The UK public are being asked to choose whether the UK should stay in the European Union or leave in the first vote on the UK's links with Europe for more than 40 years.
It has been a campaign which BBC political editor Laura Kuenssberg says is too close to call, but which has changed British politics in a number of ways.
The four-month campaign comes to a close on Wednesday, with campaigners making a last-minute appeal to undecided voters, emphasising their main arguments and encouraging their supporters to turn out to vote.
Mr Cameron, who has appeared alongside ex-PM Sir John Major and former Labour leader Harriet Harman in Bristol, has told the BBC that the decision will be irreversible and there will no coming back if the UK votes to leave.
"You can't jump out the aeroplane and then clamber back through the cockpit hatch," he told BBC Radio 4's Today.
Leaving the EU would be a "massive problem" for the UK, he said, doing "untold damage" to economic growth, jobs and family finances and hindering the opportunities and life chances of future generations.
"If I had to sum up this entire campaign in a word, it would be that word 'together'. I think together we are better able to face the challenges from terrorism and climate change, we are better able to grow our economies, better able to drive good trade deals... and I want us to get the good deals so we give better chances to everyone in our country."
Mr Cameron said he "did not believe there were any risks from staying" in the EU, saying the eurozone was beginning to recover economically and this would have an effect on levels of migration into the UK after what he said was an "abnormal period".
He also said he would lobby for further changes to free movement rules in the light of European Court rulings if the UK votes to remain and said the process of EU reform will "continue on Friday", insisting that reducing net migration was "not an unrealistic ambition".
However, European Commission President Jean-Claude Juncker, appeared to reject that option as he told reporters in Brussels "out is out", suggesting that if there was a vote to leave "there will be no kind of renegotiation", saying David Cameron "got the maximum he could receive" after months of talks which ended in February.
As party leaders past and present take the stump in the final hours, Sir John Major characterised Leave supporters as "gravediggers of our prosperity" and said Brexit may result in a "broken Britain with less importance and less influence in the world".
But Boris Johnson and other Leave campaigners said only a vote to leave the EU could give the UK the freedom it needs to set its own course, rejecting the economic forecasts suggesting the country would face a downturn following Brexit.
Speaking in Billingsgate fish market ahead of embarking on a whirlwind tour of England, the former mayor of London urged people to "believe in our country" and seize the moment.
"This is a crucial time, lots of people will be making up their minds, and I hope very much they will believe in our country, believe in what we can do," he said.
"It's time to have a totally new relationship with our friends and partners across the Channel. It's time to speak up for democracy, and hundreds of millions of people around Europe agree with us. It's time to break away from the failing and dysfunctional EU system."
In his closing campaign speech, UKIP leader Nigel Farage said it had been a "long, lonely road" for him and his party - which has campaigned for EU exit for more than 20 years - and he believed his party's supporters would "crawl over broken glass" to vote for Brexit.
He urged others yet to have made up their mind to "vote with their heart and soul", saying he wanted Britain to be a "normal country that makes its own laws and is in charge of its own destiny in the future".
But Labour leader Jeremy Corbyn insisted the UK could not tackle cross-border challenges like air pollution "on our own".
Appearing alongside Welsh First Minister Carwyn Jones, Scottish Labour leader Kezia Dugdale and Mayor of London Sadiq Khan, he said the EU was not perfect but was the best "cross-border framework for defending living standards. rights and protections for our people".
In Scotland, First Minister Nicola Sturgeon has united with her four living predecessors to back a vote for the UK to remain in the EU.
The final debates of the campaign will take place on Wednesday evening on Channel 4 and BBC One Wales. In the biggest setpiece event of the campaign on Tuesday, the two sides clashed in front of thousands of people at Wembley Arena in the BBC Great Debate.
Meanwhile, provisional figures released by the Electoral Commission on Tuesday suggest 46,499,537 people are eligible to vote in the referendum - a record number for a UK-wide poll.