Video report by ITV News Europe Editor James Mates
David Cameron has urged European leaders to address British concerns about immigration after his key demand for EU reform was branded "unacceptable".
At a crucial summit in Brussels, the prime minister used a 40-minute speech - said to be his longest yet to EU leaders - to say the "unprecedented" numbers coming to the UK could result in Britons voting to leave the union.
“The levels of migration we have seen in a relatively short period of time are unprecedented, including the pressures this places on communities and public services," he said.
"This is a major concern of the British people that is undermining support for the European Union - we need to find an effective answer to this problem."
Mr Cameron's plan to curb EU migrants' rights to UK benefits is a key part of his renegotiation proposal.
But the idea of a four-year ban on migrants claiming in-work benefits was widely criticised by his counterparts.
Angela Merkel, the influential German chancellor, said she would refuse to support anything "discriminatory" or that runs counter to the principle of freedom of movement.
European Council president Donald Tusk suggested the proposal was "unacceptable".
Mr Cameron is under intense pressure from Eurosceptics - especially within his own party - to secure substantial reforms ahead of an in-out referendum, promised before the end of 2017.