"They" are workers from other European Union countries, who have the right to live in Britain.
One billboard from the rising Euroskeptic party depicts a construction worker begging for change under the headline "EU policy at work." Another says 26 million people in Europe are looking for work, adding: "And whose jobs are they after?" Party leader Nigel Farage said the posters were "a hard-hitting reflection of reality."
Others called them alarmist and xenophobic. Labour Party lawmaker Mike Gapes accused UKIP of running "a campaign designed to sow fear and animosity and hatred towards immigrants."
Free movement of labour is a central principle of the 28-nation EU bloc. Census data shows that about 2 million people from other EU countries live in Britain. That is only slightly more than the number of Britons living elsewhere in Europe - a fact UKIP tends not to mention.
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UKIP's aim is to pull Britain out of the EU, and its message is finding an increasingly receptive audience among Britons disillusioned with mainstream parties.
One billboard image, showing the blue and yellow EU flag burning a hole in the Union Jack under the slogan "Who really runs this country?" appeals to a frustration with European bureaucracy felt across the continent.
The party, which has no seats in Britain's House of Commons, hopes to make a breakthrough in the May 22 elections for the European Parliament, where it currently has nine of Britain's 73 seats. Some polls suggest it could receive more votes than any other British party.
And it has increasingly deep pockets. The billboard campaign is funded by a wider 1.5 million pound (USD 2.5 million) donation from businessman Paul Sykes.