Dame Emma Thompson has described the scheme to send migrants to Rwanda as "eye wateringly mad and callous".
The Love Actually and Nanny McPhee star told Sky News's Beth Rigby Interviews that the government's approach does not reflect "the soul of this country".
She argued that with migration likely to become an increasingly important issue - thanks to climate change as well as wars - it was not realistic to try to "become fortresses".
Her comments about the Rwanda scheme echo those of a number of campaigners and politicians, including some in the Conservative party.
Jesse Norman, the former Treasury minister who earlier this week revealed that he was withdrawing his support from Boris Johnson ahead of a bruising confidence vote, described the Rwanda policy as "ugly, likely to be counterproductive and of doubtful legality".
The scheme to send migrants arriving illegally in the UK to Rwanda is aimed at deterring people from making perilous Channel crossings in small boats.
It has been described as "cruel and nasty" by critics and the Archbishop of Canterbury has said the plan is "opposite the nature of God".
However the prime minister argues that it is "the morally right thing to do".
Dame Emma - who has an adopted son from Rwanda - spoke about the policy during an interview with Sky News political editor Beth Rigby ahead of the release of her new film Good Luck to you Leo Grande.
She said that she did not think Britain was doing enough for "refugees across the board".
"The Rwanda scheme is eye-wateringly mad and callous," she added.
"We're exhibiting a callousness in government that I don't think reflects the soul of this country and the nature of the people in this country."
The actress said she understood that some refugees may feel that the kind of welcome being extended to Ukrainians who are white seeking sanctuary was not given in the past to Syrians fleeing war.
But she added: "At the same time, there's been a huge compassionate opening of British people.
"People in this country have all wanted to help so so much and been really severely prevented from doing so by government mechanisms and all sorts of internal difficulties with things like bank accounts - our civil society not somehow being helped to rise to the occasion.
"This is a very very complex issue but the movement of peoples not only due to war but due to climate change is going to be one of the biggest issues of our time and certainly of the 21st century.
"We better start getting our minds round how people need to move and we will have to accept that.
"We can't become fortresses, we can't put up walls anymore. It's not realistic."
The PM said when launching the scheme earlier this year that it was likely to be challenged by a "formidable army of politically-motivated lawyers".
The first flight from the UK to Rwanda under the scheme is expected to leave next Tuesday but legal challenges have already been submitted on behalf of 90 migrants.