International Trade Secretary Liz Truss has been handed a ministerial role covering equalities and policies relating to women.
The brief had been held by ex-work and pensions secretary Amber Rudd before she quit last week, but was not given to her successor Therese Coffey.
Ms Coffey, previously an environment minister, voted against legalising same-sex marriage in 2013.
Labour has accused the PM of treating the role as an “inconvenience”.
Ms Rudd dramatically quit her cabinet role on Saturday, launching an outspoken attack on the government’s approach to Brexit.
Speaking before Ms Truss’s appointment, shadow women and equalities secretary Dawn Butler had expressed concerns the role was being “undermined”.
“When the next person is appointed, they will be the tenth to be appointed to the brief since 2010,” she told the Commons on Monday.
“The post has moved departments four times, and a new minister would be the fifth I will have shadowed in just two years,” she added.
Previous holders of the post have included Theresa May, when she was home secretary, Justine Greening and Nicky Morgan.
In other appointments, Conservative Brexiter Zac Goldsmith replace Ms Coffey in her environment role, and will also attend cabinet.
The MP for Richmond, who was a prominent environmentalist before entering Parliament, ran for Mayor of London in 2016 but was defeated by Sadiq Khan following a controversial campaign.