Speaking about the affair for the first time, Thompson said she and Bonham Carter shared many personality traits which may be why her ex-husband fell for both the women.
"That is - as Mike Nicholls (the film and theatre director) once said, and I've picked it up because I think it's such a wonderful phrase - all blood under the bridge. You can't hold on to anything like that," Thompson told the Sunday Times.
Thompson, 54, has spoken about how her divorce pushed her into depression.
The 'Sense And Sensibility' star believed Branagh, whom she married in 1987, was probably attracted to both the women because they are slightly similar.
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"Oh we are. Being slightly mad and a bit fashion-challenged. Perhaps that's why Ken loved us both. She's a wonderful woman, Helena," she said.
Thompson and Branagh starred together in many successful films including 'Dead Again' and 'Much Ado About Nothing' but separated in 1995.
Thompson is currently married to actor-producer Greg Wise.