The SNP has demanded answers amid fears that controversial poster vans telling illegal immigrants to "go home" could be rolled out across Scotland.

The Tory-LibDem Coalition said it was considering extending the campaign, which has been disowned by the LibDems, across the UK.

No 10 said that there was evidence that the message advertised on the vehicles, to "go home or face arrest", was "working". But that seemed to be thrown into doubt within hours as the Home Office admitted evidence of the policy's impact had yet to be fully evaluated.

SNP MP Pete Wishart said the vans, which have so far appeared only in London, would force illegal immigrants "underground and intimidate peaceful and legitimate communities".

"It is not surprising the use of the vans has received widespread criticism," he said. "The illegal immigrants the vans are trying to target are even unlikely to have a good enough grasp of English to understand what is being communicated.

"These billboards are clearly designed to address the growing influence of UKIP and the anti-immigration plank of their agenda.

"The UK Government's priority should be fixing the immigration system and clearing the backlog so that more people take the legitimate route into the UK.

"I have written to the Home Secretary asking if there are any plans for these posters to come to Scotland.

"It seems the UK government is acting in desperation, pandering to a swing to the right in some places south of the Border, and using inflammatory rhetoric in a bid to respond to UKIPs rise in the polls."

No 10 also suggested the plan had been signed off by the Home Office ministerial "team", including LibDem minister Jeremy Browne. But at the weekend LibDem Business Secretary Vince Cable branded the campaign "stupid and offensive".

A Home Office spokesman said the impact of the vans, , was yet to be fully assessed. A poster and leaflet campaign is due to continue for another three weeks.