Rival European financial centres are already looking at how to take business from London if Britain votes to leave the European Union in a June 23 referendum, Foreign Secretary Philip Hammond said on Wednesday.

In a speech setting out the alternatives to EU membership, Hammond, who is backing an 'In' vote, said none of the outlined options were better than remaining in the bloc under a deal agreed by Prime Minister David Cameron and other EU leaders.

He said it would be "naive" to think others within the 28-country bloc were not already looking at the potential advantages of Britain being "locked out" of the European market.

"Let's be blunt, there will be people in Paris, there will be people in Frankfurt today already planning how they can seize the financial services market share in London and grow the business of their own cities if we decide to leave," he said.

Hammond said other member states would pursue their own national interest "aggressively" in any post-Brexit negotiation.

"We can talk as much as we like about friendly and amicable discussions, it will not end up that way if we snub 27 other countries who frankly have gone the extra mile to try to give Britain a deal that recognises our particular concerns ... They won't owe us any favours," he said.

The government published a report on Wednesday analysing the alternatives to EU membership, highlighting the risks of each option and warning that establishing a new relationship with the bloc from outside would not be quick or straightforward.

Britain's economy would meanwhile face years of uncertainty, Hammond said, adding that it was unlikely non-EU countries would be willing to negotiate bilateral deals before they knew what Britain's future links to the EU would be.

'Out' campaign group Leave.EU said the government's report was "full of lies and false assumptions".