Writing on Facebook on Sunday morning, he said:
President Trump's ban on refugees and immigrants from certain countries is shameful and cruel.
The USA has a proud history of welcoming and resettling refugees. The President can't just turn his back on this global crisis - all countries need to play their part.
While every country has the right to set its own immigration policies, this new policy flies in the face of the values of freedom and tolerance that the USA was built upon.
I'm pleased that the Prime Minister has now said she and the government do not agree with President Trump's policy, which will affect many British citizens who have dual nationality, including Londoners born in countries affected by the ban.
I will work with the government on behalf of Londoners affected.
As a nation that, like the USA, values tolerance, diversity and freedom, we cannot just shrug our shoulders and say: 'It's not our problem'."
Theresa May distanced herself from Trump's sweeping and chaotic ban which provoked criticism and protests. A spokesperson for the Prime Minister said the government "do not agree" on the approach on Sunday morning.