Jeremy Piven has been branded ''highly unprofessional'' by his former co-star Elizabeth Moss.

The 'Mr. Selfridge' star quit their Broadway production 'Speed-the-Plow' in 2008 because he had mercury poisoning, and the 30-year-old actress claims the entire cast was ''surprised''.

During Plead the Fifth on Andy Cohen's 'Watch What Happens Live', the 'Mad Men' star was asked: ''What, besides eating too much sushi, caused Jeremy Piven to quit 'Speed-the-Plow'?''

Elizabeth replied: ''That's a very good one. I could go on and on. Being highly unprofessional.

''We were all surprised. It came out of nowhere. He just didn't come back one day.

''I saw him like a month later at the Golden Globes. When he was supposed to be really sick.''

A judge ruled in August 2009 that Jeremy did not his breach his contract with the Broadway producers when he abruptly left the revival of the David Mamet play after his doctor said he was suffering from mercury poisoning after eating too much fish.

Meanwhile, Elizabeth was also asked about the rumours that her co-star, Jon Hamm, has ''impressive anatomy.''

She replied: ''Well, I mean, we've all seen the pictures so, I mean, I would suppose so. That's all I know.''