Eurosceptic Tory backbencher Bernard Jenkin said Brits "cannot afford to be 'little Europeans'" following David Cameron's speech on Britain's future.
Mr Jenkin said: "The Prime Minister has come to Essex to warn that we cannot afford to be 'little Englanders' - and he is right - but we cannot afford to be 'little Europeans' either, but that's where the EU is taking the UK.
"He is right that the UK's prosperity and security depend so much on what happens in the rest of the world, but wrong to suggest that the UK must stay in the EU.
"Unless there is a fundamental change in our relationship with the EU, the UK will simply have to leave the EU, so British business is free to compete".
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