Nick Clegg launched a strong attack on the government's "go home" campaign against illegal immigrants on Tuesday, suggesting that it was out of step with the "decent" centre-ground tradition in British politics.

In a phone-in on Radio 5 Live, the deputy prime minister said that he did not condone people breaking immigration laws but that he objected to the "tone" of the campaign launched by the Home Office.

He also said that it would be hard for his ministerial colleagues in the coalition to persuade him that the campaign should be extended.

Clegg, who returned to work on Monday after a break in Spain, was speaking only 24 hours after Downing Street indicated that David Cameron strongly supported the campaign and would be open to it being extended nationwide.

The campaign is running in six London boroughs featuring leaflets and posters with the message: "In the UK illegally? Go home or face arrest." The leaflets are still in circulation, but the two advertising vans that drove around displaying large versions of the poster have now been withdrawn.

"I was very surprised to see these two vans apparently driving aimlessly around north London," said Clegg, who has not criticised the posters in person before, although his aides made it clear that he disapproved last week.

"I don't think it was a very clever way of dealing with this issue of those who are living in this country who aren't entitled to."

Clegg said that he was in favour of taking a hard line against law-breaking, but that he was concerned about the tone of the campaign. "You can enforce the law effectively without instilling a tone which is unsettling to communities, particularly to mixed communities."

He said that he had discussed this matter on Monday with Mark Harper, the Conservative immigration minister, whom he described as "a very good guy". Clegg said he told Harper the Home Office should instead focus on improving border checks.

"I don't happen to think having a couple of vans driving around north London is the way of inspiring public confidence that we have an immigration system that is working properly," Clegg said.

"I have said to the Home Office in very clear terms ... that the effort should be spent not on vans drifting around north London, but actually reinstating exit checks, which I think were wrongly removed in the past and which we are committed to – and this was on my personal insistence – reinstating as part of our coalition agreement."

Some critics have claimed that the Conservatives have been promoting the hardline anti-illegal immigration message for party political reasons. Clegg hinted that he agreed, but he argued that "decent" people in the centre ground would object.

"You are going to have a Conservative politician why the Conservative party three years ago presented itself as green, compassionate and a centre-ground party, and now appears to be resorting, if you like, to some more traditional, Conservative, right-of-centre signature tunes, Europe, immigration, welfare, Europe, immigration, welfare," he said.

"Important though those issues are, I remain of the view, as a centrist politician, as someone who is determined to anchor the government in the centre ground, that actually the vast majority of decent people in this country are in the centre ground, and want to see politicians governing from the centre, not from either the right or the left."

Clegg went on: "Unless there's overwhelming evidence that this [campaign] is a really effective way of bolstering public confidence in the immigration system, and bearing down on illegal behaviour in the immigration system, I'm going to need a lot of persuasion this is something [we want to continue]."

Clegg spoke out as a YouGov poll for the Sun suggested that, by a margin of 47% to 41%, voters support the campaign.

In an article in the Daily Mail on Tuesday, Harper said that the opposition to the campaign was astonishing and that the government was simply trying to encourage illegal immigrants to leave the UK voluntarily. The posters and leaflets feature a text number that illegal immigrants can use if they want help to leave the UK.

"Let me clear this up once and for all – it is not racist to ask people who are here illegally to leave Britain. It is merely telling them to comply with the law," he wrote.

Harper said that those opposed to the campaign were effectively asking the government to condone law-breaking. "No society that encourages people to break its laws can survive. That, however, is precisely what our critics are asking us to do. But no responsible government could ever give up on its duty to enforce the law – and that is certainly not what this government will do."

The Home Office has only spent £10,000 on the campaign and officials said that, provided at least one illegal immigrant who might otherwise have been subject to enforced removal decided to leave voluntarily as a result, it could be deemed a success.

But a full assessment will not take place until later this summer. The two advertising vans were only in operation for a week, but the leaflets are still in circulation and the Home Office will be monitoring responses to the text number for another three weeks.

Governments have been running programmes for many years to encourage people who are in the UK illegally to leave voluntarily, in some cases even funding their flights home.

Over the last decade the number of so-called voluntary departures has increased considerably. There were 28,309 in the year ending in March 2013, of which 3,744 were "assisted voluntary returns" where financial assistance was provided.