The Home Secretary's special advisers have held meetings with a controversial anti-immigration group, it has been reported.
The Observer said Theresa May's staff had held three meetings with Migration Watch, which lobbies for lower immigration levels, in the past two years.
It comes amid a growing row over the rhetoric and tactics used by the government on immigration.
The vans, which have now been withdrawn after their pilot period ended, were even criticised by the Tories' tough-talking adviser Lynton Crosby, according to the Sunday Times (£).
The paper said the Australian had warned the tactic was playing into the hands of Ukip, because it focused the debate on the “tactics not the issue”.
Immigration minister Mark Harper defended the use of spot checks at London Underground stations, saying they were based on individuals' behaviour, not their nationality.
But one of the government's immigration advisers questioned the tactics.
Dr Martin Ruhs, a member of the Migration Advisory Committee, warned that Britain did not want to use tactics for dealing with migrants more familiar in countries like Singapore or much of the Middle East.
He questioned whether the headline-grabbing initiatives were effective as a policy, rather than simply being a "spectacle".
"In liberal democracies generally we don't want to do the kinds of things that are commonplace in Singapore or maybe the Middle East," Dr Ruhs told The Observer.
"You have to draw the line somewhere. Different people will draw the line in different places over what is acceptable in how you treat people.
"Obviously the Government has to do something about irregular immigration, but the issue is more complex than many policymakers believe.
"Some policies have an element of a spectacle - you want to send a signal, and those policies aren't necessarily the most effective."