According to the Inside TV blog at ew.com, Alex Kingston will be a recurring guest star on "Arrow". She will appear as Dinah Lance:
Want to meet Laurel Lance’s missing mother?
Former ER series regular Alex Kingston is joining The CW’s breakout superhero dramaArrow in a recurring guest star role. The British actress will play Dinah Lance — Laurel’s (Katie Cassidy) mom and Detective Quentin’s (Paul Blackthorne) ex-wife. Later this season Dinah will return to Starling City to make amends with her family that she abandoned years earlier following the tragic death of her daughter aboard Oliver Queen’s (Stephen Amell) yacht.
In addition to her years on ER, Kingston recently earned some geek cred in a recurring role on the BBC’s Doctor Who. While Arrow returned to Wednesday nights last week after its winter break to deliver a ratings gain, which is always great to see.