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Boris Johnson has said there is an "attractive alternative future" for the UK outside the European Union while insisting he would prefer to remain a member of a reformed organisation.

The Mayor of London and Conservative MP said the UK should consider options for life outside the EU since the cost of exit was lower than it has ever been.

He has not yet said which way he will campaign in a future referendum.

David Cameron has said an EU exit would not be "a land of milk and honey".

The prime minister has said he wants a "better deal" for the UK within the European Union ahead of an in-out referendum by the end of 2017. He is expected to set out the changes he wants to see in more detail in a letter to EU officials next week ahead of a key summit next month.

Speaking on his monthly phone-in on LBC Radio, Mr Johnson signalled that as part of the UK's negotiations he would like to see "derogations" giving the country more control over its borders in a reformed EU.

Obtaining such special provisions was "perfectly possible", he said, and if they were not granted the case for leaving the EU would become stronger.

The wider economic penalties for leaving the EU were lower than ever before, Mr Johnson said: "There is an attractive alternative future - we should be thinking about it."

Pressed on whether he would be prepared to lead the 'Leave' campaign at the in-out referendum, he refused to answer but said he would wait to see the outcome of David Cameron's talks with other EU leaders.

But he added: "In an ideal world I would like to stay in a reformed EU."