An amnesty should be offered to illegal immigrants in the UK, according to a former border chief.
Rob Whiteman argued the move would make the post-Brexit system "manageable", with the Home Office set to see its workload increase in the wake of the referendum outcome in June.
There are no official figures on the scale of illegal migration in the UK.
Mr Whiteman, who was chief executive of the UK Border Agency from 2011 to 2013, said some academic studies estimate there may be as many as 500,000 people - while campaign group Migration Watch thinks it could be more than one million.
He described the latter as "more likely" and predicted the number will continue to grow.
In an article published by the Guardian, Mr Whiteman said: "Tackling this caseload is massive.
"Scant resources are rightly focused on higher risk individuals, such as foreign-national offenders who are deported on expiry of their sentence."
He said the Government has made real progress - but better control cannot reverse the fact that the system has been abused and "some people are still here after many years".
The vast majority of cases of illegal migration are not active and staff resources could not cope if they were, Mr Whiteman added.
He said: "An appropriate amnesty would make our post-Brexit system manageable, optimise resources to focus on new cases and also strengthen border control around our lengthy coastline."
As seen in the US, this type of amnesty is "controversial", Mr Whiteman said, with arguments against including that it may send the wrong signal to future would-be illegal entrants.
"And where is a line drawn to allow many to remain and the remainder to still face potential deportation?" the article added.
It went on: "But the prize of taking a bold step will be considerable. It could shift the public mood, with people here because they are allowed to be here, and would enable the Government to tackle higher risk cases and improve its post-Brexit border operations."
An amnesty for appropriate cases would also end uncertainty for hundreds of thousands of people "in limbo" who are neither allowed to stay but likely to never leave or be deported, he added.
Mr Whiteman said the next five years will see considerable work for Home Office staff managing the border, immigration and passport systems.
He argued that it is doubtful that the department, even with more staff, will easily cope with the new workload "unless a line is drawn on much of the historic caseload, often going back for many years".
Britain is expected to seek to introduce controls on free movement following the referendum in June, with the details of any future system yet to be outlined.