By Kate Holton
LONDON (Reuters) - Britain will vote to leave the European Union in a planned referendum if it does not secure "robust, substantial and irreversible" reforms, the foreign secretary has warned, in a marked hardening of language on the issue.
Prime Minister David Cameron has promised to renegotiate Britain's EU ties ahead of a vote on membership by the end of 2017. He favors staying in a reformed EU but has said he will rule nothing out if he cannot secure reforms, which include curbs on welfare payments to EU migrants.
Most opinion polls show a majority of Britons back staying in the EU, but the gap with those wanting to leave has narrowed in recent months. The migrant crisis has boosted calls for Britain to regain greater control of its own borders.
British Foreign Secretary Philip Hammond said European leaders needed to know that Britain was not bluffing over the issue, and said Cameron's ministers would decide how to vote only once they had seen the changes proposed by Brussels.
"If we can’t get the commitments we need from our European partners on things like Britain being outside the commitment to ever-closer union, if we can’t get these things then as the prime minister has said, we rule nothing out," he told the Telegraph newspaper on Saturday.
"That’s why the package will have to be a robust, substantial and irreversible package of change with proper binding legal force. Because if we try to put to the British people a package which is anything less, we will get a raspberry from them," he said, referring to a common gesture of derision.
Splits over Europe have long plagued Cameron's right-leaning Conservatives and contributed to the downfall of the party's last two prime ministers, Margaret Thatcher and John Major.
The referendum was designed to end once and for all the matter of whether Britain, a reluctant member of the bloc since it joined in 1973, should remain in the EU.
However, EU leaders' uncertain handling of the migration crisis and their treatment of Greece over its debt woes have galvanized some on both the right and left of Britain's political divide to call for a British departure or "Brexit".
Research from the London-based Open Europe think-tank published on Friday showed that out of Cameron's 330 Conservative lawmakers, 69 are either "firmly out" or "out leaning" while 203 could vote either way.
Only 14 were firmly for staying in the EU, with 44 leaning towards staying in.
Iain Duncan Smith, a former Conservative leader and now a senior cabinet minister in charge of work and pensions, said the twin crises of Greece and European migration had hit the EU like an "out of control bulldozer".
But he added that this could work in Britain's favor by prompting EU leaders to think more fundamentally about the right to free movement across the 28-nation bloc.
The crisis "exposes the system to what we have been saying about it. It just does not function. It does not work," he said in an interview in the Guardian newspaper. "It is suddenly becoming clear that actually you cannot paper over the cracks and say ‘it’s alright, it’s only the British’.
"We still have the crisis over the euro and Greece, and then the rows over Schengen border controls are like nothing I have ever seen. It is massive."
Britain has stayed outside both the euro and the passport-free Schengen travel area.
Cameron and his ministers gather on Sunday for their annual party conference, their first since they won a surprisingly strong election mandate in May and one that is likely to be dominated by Europe.
(Editing by Gareth Jones)