EastEnders star Steve McFadden said his ex-girlfriend and former co-star's career was "destroyed" by stories published about her as a result of hacking.
The actor, who plays Phil Mitchell, was giving evidence at a hearing at London's High Court to decide the amount of compensation to be awarded in eight cases brought against Mirror Group Newspapers (MGN).
Lucy Taggart, formerly Lucy Benjamin, played McFadden's on-screen girlfriend Lisa and had a relationship with him during the time her phone was hacked.
McFadden, 55, giving evidence in relation to Ms Taggart's claim against MGN, said he remembered her being "very upset" about stories published about her and "feeling like she was being picked on and bullied".
He and the actress met on the set and were in a relationship from about 2000 to late 2003.
In a witness statement, he said they had hoped to marry, and he got "very close" to proposing, "but the biggest factor that stopped me was thinking that Lucy had a big mouth and was giving out information".
He said: "All the articles in the Mirror's newspapers that were constantly being published about us made me suspect Lucy.
"I thought she was shouting her mouth off. Now of course I know it was nothing like this and it was the Mirror's journalists stealing information from our phones."
The London-born actor said: "I now know that messages I left for Lucy must have been heard by MGN's journalists. Although I find this appalling, in some senses, I am relieved to finally know why and how so much information came out about me and Lucy.
"The sad thing is that you can't turn back the clock on what I thought and accused Lucy of at the time, which was that she was betraying me."
McFadden said in the statement: "Apart from our relationship, the Mirror's articles also had a really profound effect on Lucy's career and how she felt as a person."
He said she believed they were responsible "for putting her out of work and making her unemployable, and I think she was right".
Earlier the flight attendant ex-girlfriend of footballer Rio Ferdinand sobbed as she said the hacking of her voicemails had left her feeling "hounded" by the press.
Ms Alcorn, who complains about five articles - three of which are admitted to be the product of phone-hacking - said she was "completely shocked and disgusted" by the illegal activities at MGN which carried on in her case from 2002 until 2006.
She said that Ferdinand, who she was seeing secretly as he also had a long-term lover, became increasingly suspicious that she was talking to the press and giving away intimate details of their "pretty intense" relationship.
The hearing continues.