He framed the choice facing Britain as one between "decency" and "division", urging a fight back against what he called the politics of grievance.
This follows previous condemnations of Mr Farage by other Labour figures, including Home Secretary Shabana Mahmood, who labelled Farage's immigration policies as "racist and immoral".
The prime minister's intensified stance is partly influenced by concerns over a large far-right march and Reform UK's subsequent policy announcements on immigration.
Sir Keir aims to present an optimistic vision for a united, fairer Britain, contrasting with what he perceives as Mr Farage's divisive agenda.