Business secretary says ads urging illegal immigrants to 'go home or face arrest' are unlikely to work and create fear

A Home Office publicity campaign urging illegal immigrants to leave the UK has been described as "stupid and offensive" by Vince Cable.

The business secretary said it was unlikely to have an impact on people working in the country illegally and that instead it was designed to "create a sense of fear" about the scale of the immigration problem.

In an interview on the BBC's Andrew Marr show, Cable also appeared to endorse a report from a Commons select committee saying it was a mistake for ministers to focus on net migration figures because of the way the statistics were compiled.

Cable is the most senior member of the government to speak out publicly about the Home Office campaign, which has generated considerable controversy despite the fact that it involves just two billboards being driven around six London boroughs.

The posters say: "In the UK illegally? Go home or face arrest." The Home Office has responded to complaints about the aggressive tone of the campaign by insisting the initiative is just a pilot.

Cable said Lib Dem ministers were not consulted about the campaign and he did not expect it to be repeated. "It was stupid and offensive, and I think it is very unlikely that it will continue," he said.

He cast doubt on whether illegal immigrants have "a sophisticated grasp of English, read at a distance" and said that what made the campaign offensive was that it was designed to stoke fears that were not justified.

"It is designed, apparently, to create a sense of fear in the British population that we have a vast problem of illegal immigration," he said.

This was not true, Cable argued. "We have a problem, but it's not a vast one. And it has got to be dealt with in a measured way, dealing with the underlying causes."

He went on: "Actually, it's quite difficult being an illegal immigrant in Britain. You can't work, certainly legally. You can't have access to benefits. So the idea that there's some vast hidden army of people is almost certainly wrong."

On the same programme Sadiq Khan, the shadow justice secretary, said the Home Office campaign was "short-sighted and foolish" and there was no evidence it would work.

"I think this is David Cameron's attempt to try to win over Ukip voters," he said. "I think anybody that buys anything off the back of the lorry is foolish, and that includes this sort of silly posters."

But the Conservative MP Nick de Bois said on Sunday that the public were in favour of the Home Office trying to get illegal immigrants in the UK to leave, and that the Lib Dems should accept this.

"Are people saying after all then: 'Well, it's OK, we'll let people stay here illegally'?" he asked. "That's not what the British public wants, and I think my colleagues in coalition are divorced from the reality that the British public are pleased that … illegal immigration is something that just shouldn't be tolerated."

Cable's comments coincided with the publication of a report from the Commons public administration committee criticising the government for focusing on net migration figures. Conservative ministers are committed to getting net migration – the total number of immigrants per year, minus the total number of people emigrating – below 100,000.

Bernard Jenkin, the Conservative chair of the committee, said his committee's report showed that the net migration figure was "not fit for purpose" because it was based on a sample of just 5,000 people measured by the international passenger survey and that the margin of error was too large.

Asked about the report, Cable said that the Conservatives were "very preoccupied" with the net migration figure, but that it was not a useful guide to policy.

"This idea that you are pursing a net immigration figure is very misleading because, amongst other things, the largest number of people counted as immigrants are overseas students, who are not immigrants," he said.

"They are visitors, but under the United Nations classification they are regarded as immigrants. But they are good for the country. So obsessing about this net immigration number is not helpful."