An internal police investigation is under way after officers "abandoned" two young children with a stranger following the arrest of their mum.

Liza Butler, of Old Lodge Lane, Purley, was left to care for the toddlers, aged just one and three, despite repeatedly stressing she did not know them or their mother, who is her neighbour.

The 38-year-old has formally complained to the Met about its conduct, which left the children distraught.

Miss Butler, herself a mother of two, emerged from her house on Monday afternoon after hearing the toddlers screaming.

She initially thought their mother was being robbed until she was approached by a police officer after leaving her house.

Miss Butler said: "They asked me to watch the babies while they spoke to the mum and then asked me if I could carry on looking after them when they took her away.

"I kept saying to them I cannot keep these children, I don't know them and I don't know this woman.

"I think I've said good morning to her once. I don't know how long she's lived there, I don't know her name, she doesn't know mine. I don't know her. 

"No one was taking a blind bit of notice to me. They did not take my name. No social services came. The children were just abandoned in my care.

She added: "The most disturbing thing is they could have been left with a paedophile who could have abused them. I could have had heroin needles all around my house. It is absolutely disgusting."

Miss Butler left the toddlers with another neighbour, Natasha Brown, while she went to pick her own children up from school. The arrested woman's 18-year-old son collected them two hours later.

It is understood their mother was arrested on suspicion of obstructing officers executing a search warrant at her home.

Miss Brown, 27, said: "We were asking what we should do with the kids because they didn't know us and the policeman just said that wasn't their problem.

"I don't really know them so I thought it was wrong, but I felt obligated to watch them.

"It seemed like the police didn't really care about the kids, only about getting the people they wanted in the van."

A Metropolitan Police spokesman said: "We can confirm we have received a complaint in respect of a warrant executed in Old Lodge Lane, Purley on September 30.

"This is now subject to a local complaint investigation and as such it would be inappropriate to comment further at this stage."

The mother of the two children declined to comment.