Michael Gove has warned the UK faces a migration "free for all" unless it leaves the EU, as the Leave camp moved to exploit an admission from the Government that EU free movement of labour rules make it harder to curb immigration.
The Justice Secretary insisted potential new members of the EU posed a "direct and serious threat" to public services such as the NHS, and social harmony.
He said five countries "due to join the European Union" - Albania, Macedonia, Montenegro, Serbia and Turkey - which he warned would mean Britain's public services would not be left in a "strong position".
The intervention came as London mayor Boris Johnson came out fighting after being roundly condemned over his highly personalised attacks on Barack Obama.
And former Cabinet minister Iain Duncan Smith said the US President was wrong to suggest the UK would be at the "back of the queue" for a trade deal, claiming there were many politicians in Washington eager for an agreement with post-Brexit Britain.
Duncan Smith said immigration was "out of control" and "poorer people" had seen their livelihoods damaged by EU citizens coming to the UK.
The Home Secretary Theresa May admitted on the weekend that EU rules on freedom of movement make it harder to control immigration.
Speaking on Monday, Mrs May said Britain can cope outside the EU but it may not be better off leaving, arguing that Britain would be more secure from crime and terrorism if it stayed within the EU.