The Labour leadership contender warned that joint payments of the benefit risk trapping women in abusive relationships, as the current system allows abusers to take control of the household budget.
He appealed to ministers to split universal credit payments between couples by default, rather than automatically requiring couples who live together to make a single claim.
The warning comes as the flagship welfare reform – which rolls six working-age benefits into one payment – was hit with fresh delays, with the full roll-out now planned for 2024.
Sir Keir said that the current system of payments gives domestic abusers an “easy mechanism” to commit financial abuse, with a survey by Women’s Aid and the TUC revealing that 52 per cent of people living with their abuser could not afford to leave.
He said: “Ministers have promised that they would bring back the Domestic Abuse Bill. When they do they must take steps to ensure it provides greater financial independence in universal credit payments.
“Joint payments are unnecessary and offer an easy mechanism for abusers to coercively control their partners.
“Universal credit should be scrapped and replaced with a social security system fit for the 21st century. Until then, we must fight to reduce the damage caused by this deeply flawed system.
“Removing the risk of survivors being trapped in abusive relationships is an important first step.”
Official figures show 1.6 million women and 786,000 men experienced domestic abuse in the year up to March 2019, according to the Office for National Statistics.
Long-awaited updates to tackle domestic violence were finally brought forward in 2019 but the bill failed to complete its parliamentary stages before the election in December.
Boris Johnson promised to reintroduce the bill as part of his new legislative programme but no date has been set for when it returns to parliament.
The roll-out of universal credit has been hit by repeated controversies, with claimants facing debt, rent arrears and reliance on food banks due to delays in payments.
Adina Claire, acting co-chief executive of Women’s Aid, told The Independent: “Universal credit was not designed with survivors’ safety in mind.
“We know that perpetrators will use single household payments as a form of control, restricting access to money, which makes it even harder for a woman to leave.
“We have long been calling for the government to deliver separate payments that protect women’s financial independence, alongside wider reforms to ensure universal credit provides a safety net for every survivor of domestic abuse.”
Sir Keir took an early lead in the race to succeed Jeremy Corbyn, with shadow business secretary Rebecca Long-Bailey emerging as his nearest challenger.
Wigan MP Lisa Nandy has also secured enough support to make it onto the ballot paper, while shadow foreign secretary Emily Thornberry is scrambling to make up ground.