And he said he does not 'lie awake at night' worrying about the population hitting 70million.
Official figures show at the current rate of increase the British population will hit the milestone within 20 years, with a further seven million immigrants placing a burden on public services.
Immigration Minister Phil Woolas has pledged that the Government will not allow the population to grow to that level. But last night he was apparently undermined by his boss.
Speaking at the Home Affairs Select Committee, Mr Johnson said he would not bring in a cap because it would harm the economy, claiming the argument that immigration had made a contribution to the economy was 'irrefutable'.
'I do not lie awake at night worrying about a population of 70million,' he told the cross-party group of MPs.
'I'm happy to live in a multi-cultural society. I'm happy to live in a society where we not only welcome those coming to live and work in this country, but also where we can go and live and work in other countries.'
The Home Secretary did acknowledge the recession has made it more difficult for ministers to convince British workers who have lost their jobs that immigration is beneficial.
But his argument goes against evidence that the vast majority of jobs created in the private sector have gone to immigrants - meaning local workers have failed to feel the benefits.
In recent years, Labour has tried to head off the rise of the British National Party by giving the impression that it is talking tough on immigration.
The BNP won two seats in the European Parliament last month after it exploited fears in Labour's white working class heartlands, taking tens of thousands of votes from the governing party.
In 2007, Prime Minister Gordon Brown made his now infamous 'British Jobs for British Workers' speech at the Labour party conference. But the slogan has come back to haunt him - in particular during the dispute over the use of imported labour at the Total oil refinery in North Lincolnshire earlier this year.
A poll earlier this week found one in four Britons would like to see the population reduced by up to a third to ease overcrowding. Seven out of ten said the best way to curb population growth was to cut immigration.
The population stands at around 61million. If it hits 70million, with seven million immigrants making up most of the rise - it is the equivalent of adding the population of Sweden in just two decades, almost all of it in England.
Earlier this week campaign group MigrationWatch UK warned the balance of those settling in the country would have to be reduced to 50,000 from the current level of 237,000 every year.
The Government's points-based system is likely to cut immigrant numbers by around 8 per cent to 217,000. Tory immigration policies are likely to bring about a drop of 27 per cent to 172,000.
Last night MigrationWatch director Sir Andrew Green said: 'No wonder the political class is in such disrepute.
'Here is a new Home Secretary immediately riding roughshod over public opinion that is hugely opposed to the mass immigration which this Government is encouraging.
'For a start, how is a government that is broke going to pay for all the houses, schools and hospitals that an extra seven million immigrants will need in the next 20 years?'
Former Labour Minister Frank Field, who runs a cross-party group called Balanced Migration, which campaigns to limit the number of immigrants to manageable levels, was dismissive of the Home Secretary's claims not to lie awake at night. He said: 'It must be a misquote because it should be.
Green Arrow gives a harrowing insight to the truth here