The star, who created and stars in the show, now in its fifth season, gushed about Allison and described her performance on Sunday's episode (27Mar16) as "stunning".

Taking to Instagram to post a still of Allison's newlywed character Marnie Michaels from the show, she wrote: "Been waiting for months to show you all @aw's stunning work in tonight's episode. Adding "One of the great privileges of my life has been watching Allison grow and stretch as an artist over the past five and a half years.

The compliments come as Lena, 29, recently shocked fans by announcing that the sixth season of the drama would be the last, with filming set to go underway this summer (16).

Allison, 27, in particular, was vocal in expressing her sadness that the show was coming to an end, after she admitted on the U.S. breakfast show Today that she was going through the "grieving process" as her co-stars had become "like family."

"It's so weird. I always knew it was going to end, but I've been texting everyone associated with the planning and the scheduling of the show, being like, 'Could you just put it off a little bit longer?'"

"I don't want it to end because it's like, once it starts, it's just the beginning of the end and then, unlike every other year, I don't get to say to everyone, 'OK, I'll see you next summer,'" she continued, before joking, "It's summer camp ending, and you know, my counsellor years are over. I've stretched it to the max; it's creepy if I hang around!"

In her recent Instagram post, Lena echoed Allison's sentiments, revealing that the star has become like family to her over the last five years.

She continued: "She (Allison) has given every ounce of herself to this character, operated with complete trust and vulnerability and taught me so much in the process- about Marnie and about myself. Thank you so much for taking the (literal) dive, Al. You are, quite simply, my sister."

The fifth season of Girls began airing on Sunday nights in the U.S. from February this year (16).