(CNN) Embattled British Prime Minister Theresa May lost a key ally late Sunday with the resignation of Home Secretary Amber Rudd, who quit after claiming she "inadvertently misled" government over targets for the deportation of illegal immigrants.

Rudd resigned Sunday amid a growing scandal over the government's mistreatment of the so-called "Windrush generation," men and women from the Caribbean who arrived in Britain in the 1950s and 60s, but in recent years have been declared illegal immigrants despite having lived in the country for decades.

She becomes the fourth high-ranking minister to resign from May's government in the past six months. It puts further pressure on May, already struggling to maintain government unity over faltering Brexit talks.

Rudd had been under pressure to step down over her involvement in the affair, following allegations that members of the Windrush generation -- so named after the ship that had brought hundreds of Caribbean migrants to Britain -- had recently been refused medical care, denied housing and threatened with deportation.

On Monday, Rudd was due to face a fiery session in the House of Commons. Instead, May is expected to announce Rudd's replacement, an influential role that comes at a delicate time for the UK government as its negotiates Britain's exit from the European Union.