A senior European Union official will step up his criticism of the UK's stance on immigration, accusing politicians of "pandering" to xenophobia.

Employment Commissioner Laszlo Andor's previous criticism of David Cameron's "nasty" looking restrictions on benefits for foreigners led the angry Prime Minister to lodge a formal complaint.

But the Hungarian will use a visit to Britain to warn Mr Cameron he cannot "cherry pick" EU freedoms or base policy on "perceptions, gut feelings or anecdotes".

Mr Cameron has come under fire from several fronts over his push to curb the free movement of workers, which he says is too often abused by people seeking handouts not jobs.

A three-month delay before EU immigrants can claim out-of-work benefits was introduced amid claims the lifting of restrictions on Bulgarian and Romanian access to the UK would result in a spike in arrivals.

Mr Andor called that an "inappropriate over-reaction" which risked Britain looking "nasty" - comments Mr Cameron told Commission president José Manuel Barroso were out of order from an unelected official.

But Mr Cameron is also pushing for more fundamental curbs such as limiting the free movement of workers from new member states until they hit a certain level of GDP per head.

He says the Commission lacks "practical, good, Conservative common sense".

In a speech at the University of Bristol Mr Andor will say politicians are avoiding "inconvenient truth" that most migrants move to work and are an "asset" to economies like the UK's with an ageing population.

"The truth, and indeed it might be 'inconvenient' for some, is that the vast majority of people who move from one EU country to another do so in order to work. They don't do it in order to claim benefits.

"These workers are in fact of considerable benefit to the economies, and to the welfare systems, of the receiving countries.

"In the specific case of the UK, as confirmed recently by the Office of Budget Responsibility, public debt would be much higher in the future without immigration because of the ageing population.

"Do people in the UK really want to pay higher taxes instead of immigration?

"No Member State has given the Commission any factual evidence that so-called benefit tourism is systematic or widespread.

"In the case of the UK, I have been asking for such evidence to back up their claims for over two years but all that we have received is a serious of anecdotes about cases of criminal fraud and vicars performing sham marriages.

"I firmly believe that the debate on labour mobility should be based on facts rather than perceptions, gut feelings or anecdotes.

"For some, this may be an "inconvenient" approach, but I do believe people deserve to be given the facts. And politicians should be responsible enough to talk about facts, rather than to pander to prejudice, or in the worst cases, xenophobia."

In a direct swipe at Mr Cameron's wider reform drive - to which he is on a diplomatic mission to recruit supporters across Europe - Mr Landor will say freedom of movement is " just one of the four freedoms on which the EU's Single Market is based, along with free movement of goods, capital and services.

"Member States cannot cherry pick which of the freedoms they want and leave the rest because the EU is based on a balance of mutual interests."

The Prime Minister plans to renegotiate the UK's relationship with Brussels ahead of an in/out referendum by the end of 2017 if he is returned to office after the next general election.

Work and Pensions Secretary Iain Duncan Smith has indicated that he would like EU immigrants to have to wait for up to two years to claim benefits - rather than the three month period and had support for such a dramatic extension from Germany, Italy and the Netherlands.

London Mayor Boris Johnson has also mooted a similar move but Downing Street insists it would require treaty change and could not be done immediately.

European Commission vice-president Viviane Reding - who is also visiting Britain this week - has accued British politicians of peddling "myths" about an influx of EU migrants.

Ms Reding - who is due to take part in a public debate on the EU today ahead of a major speech - said it was "simply not true" that there was an "invasion of foreigners" who were stealing jobs and draining welfare and health resources.

Polish prime minister Donald Tusk has also protested about Mr Cameron singling out his countrymen in an announcement that he wants to change a system under which migrants from EU states are able to claim UK child benefit for offspring living in their home country.

At a recent Brussels summit Mr Cameron defended his stance.

"It is completely consistent to be in favour of widening membership...but at the same time understanding the very real concerns which people have about migration and which we need to address," he said.

"I would argue that is practical, good, Conservative common sense and frankly that is something the European Union needs a lot of."

Mr Andor's planned remarks infuriated Work and Pensions Secretary Iain Duncan Smith.

"Once again the European Commission is showing how completely out of touch it is," a source close to the cabinet minister said.

"Hardworking British taxpayers want to know the benefits system is there as a safety net for people in real need.

"It shouldn't be a cash machine for people who have made no contribution to this country first.

"These attempts to interfere in the British welfare state are frankly tiresome.

"We know that the majority of those who come to the UK do contribute, and we welcome that.

"But it is absolutely right that the UK government take action to ensure that the benefits system is protected from exploitation."

Conservative Party chairman Grant Shapps said: "Why Mr Andor thinks it's his job to criticise the Government's actions to get a grip of our borders is beyond me.

"Rather than resort to nasty insults and cheap political jibes he should support the difficult long term decisions we're taking to support hard-working families and stop abuse of our public services."