LONDON (REUTERS) - Italy's prime minister said on Tuesday that there was a "huge risk" that Britain would leave the European Union (EU) in the next few years and he urged other countries to work to reduce the prospect of an exit.

"We are convinced that in the European Union countries, we are (underestimating) the risk of having the UK out of Europe.

In my view, it's a huge risk," Mr Enrico Letta told an audience in London after making a speech.

"We have to be very cautious... and we have to prepare a discussion on trying to prevent this risk," he said in response to a question.

British Prime Minister David Cameron, who is due to meet Mr Letta on Wednesday in London, has promised to hold a referendum on Britain's continued membership of the EU by the end of 2017, if his Conservative party returns to power at elections in 2015 and he can win concessions on Britain's relationship with the EU.

Surveys suggest Britons are divided on staying in the EU, with slightly more favouring an exit, but many remain undecided.

Mr Letta said that an EU without Britain would be less committed to free markets and less of a global player.

Italy is due to hold the 28-nation bloc's presidency in the second half of 2014.

In his speech, Mr Letta said moves towards more integration in the euro zone should not threaten the EU's single market or leave countries outside the single currency area less comfortable with their membership of the bigger union.

"We will work together to give good reasons for the UK to see its future in a reformed European Union," he said, listing less regulation, a deeper single market and new international trade deals as attractions for Britain.

Mr Letta was speaking at the Chatham House think-tank in London at the start of a two-day visit to Britain.