A Liberal Democrat Home Office minister says his party will not agree to further use of vans telling illegal immigrants "to go home or face arrest".
Jeremy Browne told the party conference in Glasgow: "We didn't agree those vans... and we won't be agreeing any proposals to bring them back."
The Home Office's week-long pilot saw vans driving around parts of London.
Lib Dem leader Nick Clegg said he had been surprised by the scheme and said Mr Browne had not approved them.
The Home Office says that the pilot worked, although a full assessment has yet to be completed and published.
The department said last month that it made no apology for carrying out immigration enforcement operations, but ministers were deciding whether future campaigns would be conducted.
The vans have also faced an inquiry from the Advertising Standards Authority, and a complaint from a French typeface designer whose copyrighted font was used on the vans.