'It's risky' So why are couples choosing an open marriage?

"We had an arrangement, be discreet and don't be blatant. There had to be payment, it had to be with strangers," sings Lily Allen in her surprisingly candid and detailed album about her open relationship with her ex-husband.

- Jessica Murray Social affairs correspondent

The album has catapulted the concept of non-monogamous relationships into the spotlight, and couples therapists have reported an increasing number of their clients are choosing to go down this route.

But as Allen's album makes clear, while open marriages, or consensual non-monogamy, may work for some, they can also go very wrong - and there are some common pitfalls to avoid.

"It's a risky business emotionally," said Katherine Cavallo, a psychotherapist and spokesperson for the UK Council for Psychotherapy.

"I'm seeing it more and more in the work that I do, but how it manifests itself varies hugely.

"It's normal for feelings of jealousy and insecurity to emerge, and those need to be responded to. The existing relationship, the attachment between the couple, needs to be maintained as well.

"And things can always change. It has to be an ongoing process in which things are continually reviewed to make sure it remains consensual."

Communication, consent and trust are crucial, she said, and if agreed boundaries aren't adhered to, it can lead to "significant emotional and relational trauma".