Nigel Farage has said immigration must be a key issue in the EU referendum campaign, claiming Europe's asylum policy has "opened the door to an exodus of biblical proportions".
The UKIP leader said those wanting to downplay the subject in the run-up to the vote, fearing it would strike too divisive a tone, were wrong.
The EU had "sent a message" that millions of migrants were welcome.
UKIP will launch its own campaign to leave the EU, not join other groups.
His comments come as record numbers try to enter Europe and in the wake of the deaths of 71 people in a lorry in Austria last week - most of whom were thought to be Syrians fleeing the country's civil war.
The tragedy has prompted calls both for more rights for migrants and for border controls to be tightened up across Europe.
'Opened the door'
Mr Farage said the authorities should be making it "more difficult" for people to cross borders, and said the EU had "lost sight" of the difference between genuine refugees fleeing from war and persecution and economic migrants wishing to better their lives.
"The problem we have now is if you look at the definition of the EU's asylum policy, it includes anybody who comes from a war-torn country and it even includes people leaving extreme poverty," he told Radio 4's Today programme.
"The EU have sent a message that anybody who comes across the Mediterranean or comes through Turkey, once they have set foot in an EU country they will be accepted. That's sent a message to hundreds of thousands of people that they can come.
"The problem we've got is potentially we've opened the door to an exodus of biblical proportions, meaning millions and millions of people," he added.
Also speaking on Monday, Labour leadership campaigner Yvette Cooper said EU nations had failed to work together to respond to the challenge of the current crisis and had a responsibility to separate asylum policy from debates about managing immigration.
Mr Farage said UKIP would play a "unique role" in the referendum on the UK's future in the EU, to be held by the end of 2017, but again insisted he would not lead the official No campaign urging withdrawal.
The Electoral Commission has yet to decide which group to designate as the official No campaign for the referendum, with two groups currently vying for the role. Designation brings with it the benefits of higher spending limits, television broadcasts and a grant.
Mr Farage said UKIP, which will launch its own campaign on Friday, would work alongside whichever "umbrella" group was chosen as well as Eurosceptics in other parties.
"The unique role that UKIP can play within this is that we have 50,000 members, hundreds of branches across the country and we can do the ground campaign."
Mr Farage suggested UKIP's performance in the 2014 European elections, when it topped the polls in the UK, showed it could deliver nearly two thirds of the votes needed to win a referendum on EU membership.
He added: "It is no different to the Yes campaign... already the SNP have said they will do their own thing and not join up with anybody and do their own thing. It is the same on both sides."
Analysis by political correspondent Robin Brant
Nigel Farage believes UKIP has the structure in place to mobilise thousands of activists that could prove invaluable to any No campaign, placing his party is at the heart of the effort.
Mr Farage also thinks the other No camps lack his combination of political nous and reach outside of Westminster. Nonetheless he paints a picture of harmony on the No side.
He says he speaks to senior figures in the other camps several times a week. He is also inviting them to UKIP's conference in Doncaster at the end of September. He wants to see them up on the stage addressing his faithful.
What of the Yes camp? Easyjet chief executive Carolyn McCall had been thought to be the person being eyed up to front the campaign - but Business for New Europe have now said she will not lead it.
Behind the scenes, early funding is coming from Lord Sainsbury and there is already a cross-party consensus in place with Peter Mandelson, Danny Alexander and Damian Green the self-appointed political figures.
Strategy is quite advanced. University students arriving for the start of term are likely to face a concerted effort from a Yes campaign that wants to take advantage of political activists already in place on campuses across the country.
Mr Farage said he hoped the two other groups would come together to fight the No campaign as one, which he suggested could be fronted by a non-politician, such as someone from the worlds of business and entertainment.
He added: "What I do know is the Yes campaign is very active. Over the course of the last few months we have seen Richard Branson, Tony Blair and Lord Mandelson. They are out there campaigning and we need to get cracking."