'I received a text message saying, "I'm really sorry, Mum, I do love you, but we don't want to be father and daughter any more. We're having a relationship" '

When I met Mike, my second ­husband, my youngest ­daughter from my previous marriage was 12 years old. Mike raised Lisa as if she were his; she called him Dad and even changed her surname by deed poll to match ours. He and I went on to have three children of our own. In the eight years Mike and I were ­together, he always treated Lisa like a daughter. Or so I thought.

Four and a half years ago, Lisa ­decided to work abroad. My ­husband reacted badly, which I couldn't ­understand. She was 20 and, with three younger children, the house often seemed cramped. I'd been e­ncouraging Lisa to find a place of her own, which Mike always ­resisted. He'd say, "She's our daughter, she can stay as long as she wants."

Once she'd gone, Mike became withdrawn and moody. He said he needed a break and went to visit Lisa. A week later he returned, bringing her with him.

That was my first inkling some­thing wasn't right. Then I thought, "No, this is your husband, your daughter, you're losing the plot." But the uncertainty kept ­niggling at me. Eventually I confronted Lisa, who stormed out of the house. Four hours later, I received a text message saying, "I'm really sorry, Mum, I do love you, but we don't want to be father and daughter any more. We're having a relationship."

In shock, I phoned my husband. He said he loved me, he loved the children, but he loved her, too. He said he couldn't be with either of us, because he didn't know which way to turn. But he didn't come home that day. He never came back again.

The news tore through our family, provoking very different reactions. People didn't know what to say. Some turned away. Others became angry. My older children went through hell. I virtually had a nervous breakdown – I'd lost my husband and daughter, two people I'd also classed as very good friends.

How long had it been going on? Friends have since told me Mike and Lisa always seemed ­unusually close; some suggested he had been "grooming" her; others said she'd had a crush on him even as a 12-year-old and set out to get him. I don't believe Mike abused my daughter – I think their sexual relationship began when he started seeing her as a woman rather than a child. Nevertheless, he was someone she'd called Dad, and he was old enough to have been her father.

Three days after they left, I took everything from Lisa's room and up to the top of the garden – clothes, personal things, even bedding – and burned it. For six months, I ­really blamed only her.

Mike and I had a horrendous court battle over the house, which I won – he wanted a share of the money to set up a home with Lisa. He stopped seeing the children after I insisted he could visit them only on his own. It seemed he was trying to recast Lisa as a stepmum to her own half-siblings, which was upsetting and confusing.

Over time, I've put my life ­together. I went back to university and started training to become a social worker. I felt as if I'd been to hell, and was just about free. Then, three weeks ago, Emma, one of my older daughters, gave birth to my first grandchild.

I was sitting on the ward, cradling the newborn, when I glanced up and saw Lisa and Mike at the ­reception desk. They hadn't come to visit Emma – it's unlikely they even knew she was in the hospital. Lisa was heavily pregnant.

No one spoke. The ward sister, who I know well, ran over and said, "I'm so sorry. I thought you knew." She pulled a curtain across and took me into a private room, where I broke down.

Afterwards, I sent Lisa a note. It just said, "I hope you're OK and everything goes well with the baby. Mum." I didn't expect a reply, and I didn't get one, but regardless of what's happened, she's still my daughter. If she knocked on the door tomorrow, I wouldn't turn my back on her.

I will never have contact with my second grandchild. I wouldn't be welcome, I know that. It would mean Mike and Lisa having to ­confront what they've done, and what this child means. I feel sorry for the child, too, because at some point, years down the line, he or she will need to know the truth, and how will that feel?

As told to Chris Broughton. All names have been changed.

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