Britain's continued membership of the European Union is contingent upon it being allowed to stop migrants from the bloc tapping into its relatively generous welfare system, Prime Minister David Cameron will warn on Friday.
In a speech designed to breathe new life into his campaign to be re-elected in May next year, Cameron will say he wants EU migrants in work to have to wait four years before they can access welfare benefits and for unemployed EU migrants not to be eligible for any help from the British state at all.
With polls showing immigration is voters' top concern, Cameron is under pressure to take a tougher line on the issue. Many of his Conservative lawmakers fear the rise in popularity of the anti-EU UK Independence Party, which this month won its second seat in parliament, threatens their re-election chances.
The speech is likely to upset EU partners such as Poland which has suggested such measures would be discriminatory. But Cameron will not call the EU's freedom of movement rules into question as some EU leaders had feared and will not advocate migrant quotas or an "emergency brake" on inflows.
If re-elected, he has promised to renegotiate Britain's ties with the EU before giving Britons an in/out membership referendum in 2017.
He will make it clear on Friday he thinks his renegotiation will succeed. But he will also drop his strongest hint yet that he may campaign for Britain to leave the bloc if he fails.
"I will negotiate a cut to EU migration and make welfare reform an absolute requirement in renegotiation," Cameron will say, according to advance extracts released by his office.
"If I succeed, I will, as I have said, campaign to keep this country in a reformed EU. If our concerns fall on deaf ears and we cannot put our relationship with the EU on a better footing, then of course I rule nothing out."
If implemented, his proposals, which are designed to make Britain a less attractive place for migrant labour, would affect over 300,000 EU migrants, many of them working in low-wage low-skilled jobs.