BBC period drama Upstairs Downstairs will feature a lesbian plot line when it returns this spring, one of the show’s writers has revealed.

Emilia Fox will play Lady Portia Alresford, a former lover of Blanche Mottershead, portrayed by Alex Kingston, best known for her role as ER’s Elizabeth Corday.

The first series of the Upstairs Downstairs revival was closely watched as the BBC’s answer to ITV’s seemingly unstoppable Downton Abbey.

The latter briefly tackled gay relationships in high society with a dalliance between the Duke of Crowborough and a footman of the Crawley family in its first series.

While that relationship was kept quiet, the lesbian affair in the inter-war drama Upstairs Downstairs is made public.

Writer Heidi Thomas told the Mirror: “These are two brilllant, intellectual women who have a deeply romantic friendship in the tradition of the great Edwardian romantics, which gives each of them endless stimulation and satisfaction.”

The “covert and difficult” relationships, she added, “causes a bit of a stir when it all comes out, but Blanche finds support and sympathy from an unlikely quarter”.

“She is quite a radical, very used to a free existence, and doesn’t understand why she would have to conform to the beautifully-kept world they’ve created for themselves.”

The series is due for broadcast in the spring.