London will fall like the Greek city of Sparta if it turns away immigrants like the ancient militaristic regime which "kicked people out", Boris Johnson has said.
David Cameron's pledge to bring down immigration into the tens of thousands was a"policy failure" and should never have been attempted, the Mayor said.
"Look at Athens and Sparta," Johnson told the Telegraph. "Athens was an open city and Sparta kicked people out. Go and look at the ruins of Athens and Sparta now and ask which of the two cities made the greatest contribution to civilisation. Look at the greatness of the American economy."
The government appears to have admitted defeat on the Tory pledge to bring annual net immigration below 100,000 by the general election.
Official figures showed net migration - the number coming to the UK for at least a year, minus the numbers leaving - rose 58,000 to 212,000 in the year to September 2013.
Cameron has rejected calls to drop the target, arguing it is still "achievable" but refusing to offer a "cast iron guarantee". Home Secretary Theresa May has conceded it has become "more difficult".
"There was a policy failure of politicians, saying they wanted to control immigration which they couldn't do because of free movement," Johnson said, speaking at Investment 2020, which encourages companies to hire young people from non-financial backgrounds. "I'm one of the politicians willing to stand up for immigration."
But johnson said that efforts to celebrate and promote multiculturalism had meant many migrant communities had not felt the need to integrate. "I wan t to see people proud of Britain, we have to insist on that. We went through a long period of cultural laissez-faire, where we didn;t understand that they want to speak English."
Sparta was one of the most powerful Greek city-states, famous for its military mastery, the home of the best and most feared fighters in Greece. From the age of seven, male citizens served in the military and lived in barracks, with disabled children or those perceived to be weak killed shortly after birth.
Foreigners did live in Sparta, but were routinely forcibly expelled by their hosts.