* British PM unveils new plans to deter immigration
* Cameron under pressure from anti-immigration party
* EU executive warns Cameron some rules non-negotiable
By Andrew Osborn
LONDON, Nov 27 (Reuters) - Prime Minister David Cameron unveiled plans to limit European Union migrants' access to welfare in Britain and said he wanted eventually to restrict migrants from poorer EU states relocating to richer ones, stirring a row with the European Commission.
Cameron's Conservatives risk seeing their vote split at European elections next year and at a national election in 2015 by the anti-immigration UK Independence Party (UKIP) and he is under pressure from his own party to get tough on the issue.
Trailing in opinion polls, Cameron said he shared deep concerns about a possible influx of Romanians and Bulgarians next year when EU restrictions on those two countries expire, something UKIP says could lead to millions of new migrants.
"The EU of today is very different from the EU of 30 years ago," he wrote in an article for the Financial Times.
"We need to face the fact that free movement has become a trigger for vast population movements caused by huge disparities in income. That is extracting talent out of countries that need to retain their best people and placing pressure on communities."
Cameron said he planned to change British law so that new EU migrants would have to wait three months before they could obtain unemployment benefits.
Newcomers would not be eligible for housing benefits and would lose the right to unemployment benefits after six months unless they proved they had a realistic chance of finding work.
His longer term idea of limiting free movement of migrants from poorer EU states would form part of his renegotiation of Britain's membership of the EU, he said. Cameron has promised to reshape Britain's EU ties before an in/out referendum after 2015 if he is re-elected amid scepticism about the EU.
His idea of reforming the EU's freedom of movement rules would need to be negotiated with other member states and could face a legal challenge from the European Commission.
The EU executive has not said whether or not it will take legal action, but has made it clear it would "rigorously" oppose attempts to restrict freedom of movement, a central tenet of the EU's 500-million-people single market.
The Commission told Britain on Wednesday that EU freedom of movement rules were non-negotiable and that London had to accept them if it wanted to remain in the bloc's single market.
"If Britain wants to leave the single market, you should say so. But if Britain wants to stay a part of the single market, free movement applies. You cannot have your cake and eat it, Mr Cameron!" Viviane Reding, vice-president of the EU executive, told Reuters.
Cameron's longer-term idea of restricting freedom of movement for migrants from poorer countries included capping the annual number of EU migrants or withholding full freedom of movement rights until a country achieved a certain gross domestic product per head, he said.
"Britain, as part of our plan to reform the EU, will now work with others to return the concept of free movement to a more sensible basis," he wrote.
British officials said Germany, Austria and the Netherlands shared Cameron's concerns and were potential allies.
Britain's previous Labour government waived transitional controls for migrants from new EU members states, something Cameron called a "monumental mistake".
That, he said, meant 1 million people from central and Eastern Europe were now living in Britain.
Laszlo Andor, the European Commissioner for Employment and Social Affairs, said the kind of "unilateral rhetoric" Cameron was indulging in on immigration was unhelpful.
"This is an unfortunate over-reaction. We would need a more accurate presentation of the reality, not under such hysteria which sometimes happens in the UK," he told BBC radio.
"Unilateral rhetoric ... is not really helpful. It risks presenting the UK as a kind of nasty country. We have to look into the situation collectively and act proportionately."