Steve McFadden, who plays Phil Mitchell in EastEnders, appeared at a hearing at London's High Court to give evidence in Lucy Taggart's case against Mirror Group Newspapers (MGN).
"All the articles in the Mirror's newspapers that were constantly being published about us made me suspect Lucy"
Lucy Taggart, formerly Lucy Benjamin, played McFadden's on-screen girlfriend Lisa - and also had a real-life relationship with the actor during the time her phone was hacked by MGN.
The 55-year-old said that he had hoped to marry Lucy and was "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."
McFadden added: "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 added that she believed that MGN were responsible "for putting her out of work and making her unemployable, and I think she was right".
The hearing continues.