Nick Clegg has again attacked Theresa May's department for dragging its heels over introducing exit checks, marking off a week of increased tension between the deputy prime minister and home secretary.
Clegg used his phone-in show on LBC to say he was "frustrated" about the lack of progress in introducing the immigration checks.
Yesterday he used a Whitehall press conference to tell journalists he was "concerned" at the lack of movement on the scheme and would send David Laws to ensure the Home Office was pressing ahead, even though he seems to have given up on having completed it by 2015.
"I'm not going to pretend to you that exit checks will be restored in full, according to the Home Office's present plans, by the end of this parliament, but I hope we will be able to demonstrate very significant progress in doing so, because it was a mistake lifting these exit checks in the first place," he said.
"I have for some time been concerned with the urgency with which the Home Office is seeking to implement the coalition agreement commitment that I personally insisted on, that exit checks should be restored.
"For quite a long time now, I have felt it has taken longer to fully implement exit checks than I originally hoped."
Exit checks would allow the government to have a better sense of visitors who had overstayed their visas, by showing the status of those leaving the UK.
Clegg has latched onto the issue as a way of demonstrating his tough credentials on immigration – a policy area which the Liberal Democrats have usually struggled with.
The Home Office's most senior civil servant, permanent secretary Mark Sedwill, told the home affairs committee last month that he still believed he could hit the 2015 target.
"The objective is to have exit checks through e-borders and other mechanisms by 2015, so essentially by the end of the parliament," he said.
The Clegg/May spat comes after the deputy prime minister expressed reservations about the so-called 'racist vans' which drove around London with a billboard telling illegal immigrants to "go home".
Pressed on the issue during today's phone-in show, Clegg said he did not object to the message but thought the scheme was an inefficient use of resources.