Marsha de Cordova, the shadow equalities secretary, has resigned from Keir Starmer’s shadow cabinet with immediate effect citing a desire to focus on her marginal constituency.

The Guardian understands that the shadow women’s minister, Charlotte Nichols, has also stepped down for personal reasons unconnected to any political dispute. Nichols is expected to return to the frontbench in a different brief.

De Cordova’s resignation will leave Starmer with a significant role to fill ahead of Labour’s conference next weekend, with a replacement expected to be announced shortly.

She announced her resignation in a tweet saying: “It has been an immense privilege to serve as the shadow women and equalities secretary for the past 17 months. It therefore comes with much sadness that I am resigning with immediate effect.

“Having only been elected in 2017 for the historically marginal constituency of Battersea, I would like to focus more of my time and efforts on the people of Battersea. I will continue to support Keir Starmer from the backbenches.”

Starmer said he wanted to thank De Cordova for her work, including “highlighting the disproportionate impact the pandemic has had on black, Asian, minority ethnic and disabled people … Marsha has also laid the foundations for a new Race Equality Act that Labour would introduce to tackle the structural inequalities which have existed in our society for too long.”

One senior MP said the role would be difficult to fill, citing tensions at a senior level over the party’s stance on transgender rights.