'But the more generous you are, the more word gets around about this — which in turn motivates more people to leave Africa.
'Germany cannot possibly take in the huge number of people who are wanting to make their way to Europe.'
Mr Gates praised Chancellor Merkel's commitment to spending 0.7 per cent of GDP on foreign aid as 'phenomenal', and asked other European leaders to follow suit.
But he added: 'Europe must make it more difficult for Africans to reach the continent via the current transit routes.'
A German government report which leaked to the Bild newspaper suggests there could be up to 6.6million people trying to get into Europe, including 2.5million waiting to cross from North Africa.
It is thought that 2,000 people have lost their lives making the crossing since the start of the year.