A mother and her new boyfriend are standing trial accused of murdering and sexually abusing a two-year-old girl who suffered weeks of alleged violence before her death.

Two-year-old Isabelle Welsh collapsed at her home in Hartington Close, Thornaby, Teesside, after what prosecutors describe as a massive head injury. She could not be revived despite frantic efforts by paramedics and specialist hospital staff.

Her mother, Alexandra Walker, 25, and her partner, 22-year-old Harrison Simpson, deny murder, allowing the death of a child, sexual assault and child cruelty. The case is being heard at Teesside Crown Court.

The court was told the pair had only recently begun a relationship the previous summer, with Simpson becoming a regular presence in the small two-bedroom property and spending a great deal of time with Isabelle.

Jurors heard that Walker dialled 999 after Isabelle collapsed. Paramedics arrived to find the toddler at the bottom of the stairs, pulseless and critically ill. Prosecutor Richard Wright KC said she was covered in bruises, particularly to her head, neck, abdomen, back and genital area. Her nappy contained blood and there was vomit on her face.

Isabelle was rushed to hospital but died in the early hours of the following morning. A post-mortem examination later revealed fractures to 21 bones, including injuries consistent with forceful gripping, and extensive bruising.

The prosecution alleges Isabelle had been violently shaken, her spine over-extended and her head struck against a hard surface such as a wall or floor. Wright told the jury that her fatal head injury was the culmination of a sustained campaign of violence.

Jurors heard that Walker took Isabelle to a GP and then to hospital 11 days before the child died, when a fractured leg was discovered. Despite concerns raised by some medics, Isabelle was discharged back into her mother’s care. Prosecutors say the leg fracture was no accident and that Walker waited around two weeks before seeking help.

The Crown argues that in such a small home both defendants had ample opportunity to harm Isabelle and that each must have been aware of the abuse. Wright said their relationship was unhealthy, involving alcohol and drugs, and that Isabelle’s care deteriorated before she was allegedly subjected to regular violence.

Unusually, CCTV cameras inside Walker’s home captured footage that will be shown to the jury as the trial continues.