At an EU referendum event, the Leave campaigner advocated an amnesty for illegal immigrants who have been in Britain for 12 years.
Mr Johnson also called for an Australian-style immigration points system would enable Britain to "take back control" of its immigration policy.
A Brexit could lead to a host of other states in eastern Europe following the UK out of the bloc, Luxembourg's foreign minister has said.
Jean Asselborn said that David Cameron had committed a "historic mistake" by sanctioning a national referendum on the issue.
He warned a decision to lead could rigger a wave of instability and defections in an interview with the German Tagesspiegel am Sonntag newspaper.
He added that even a vote to stay will not lay the issue to rest as it will not deal with the "negative attitude of the British" towards the EU.
In a 2013 interview, Boris Johnson says he wants Britain to be able to 'trade freely with our European friends'.
The Mail on Sunday and the Observer come out for remain while the Sunday Times and Sunday Telegraph back a Brexit.
David Cameron and George Osborne have issued an impassioned appeal for a vote for Remain, warning that Brexit would be the "big mistake"