Jeremy Corbyn is accusing Theresa May of being personally responsible for the Windrush generation controversy by setting a deliberately unreachable bar with her "hostile environment" immigration policies.
The Labour leader is telling the party's Welsh conference that "lives have been ripped apart" by the crackdown.
Mr Corbyn is set to tell the Llandudno gathering on Sunday: "This week, something rotten at the heart of Government has come to the surface.
"The Windrush scandal has exposed how British citizens who came to our country to rebuild it after the war have faced deportation because they couldn't clear the deliberately unreachable bar set by Theresa May's 'hostile environment' for migrants.
"And it's not as if they weren't warned. At the time the Tories were pushing their hostile environment policy through Parliament some of us, sadly far too few, warned about the consequences for those born in the UK and those born abroad alike.
"So now we're seeing those consequences in a string of harrowing human stories. People's lives ripped apart because of the personal decisions and actions of Theresa May and her Government.
"The mother of Windrush citizen, 57-year-old Dexter Bristol, who died last month after being classed as an illegal immigrant, has said she believes the stress of the experience contributed to his death.
"She argued, powerfully and convincingly, This is racism. He was the victim of their policies. There was a lot of racism when I came here. This is worse, this is the Government'."