Senior detectives failed to properly investigate the case of a toddler who may have been sexually abused before she died, a police watchdog has concluded.

Poppi Worthington was 13 months old when she died suddenly and unexpectedly in Barrow-in-Furness, Cumbria, in December 2012.

A family court judge subsequently ruled that her father, Paul Worthington, had probably sexually assaulted her. Mr Worthington, however, has strenuously denied any wrongdoing and has never been charged with any offence.

Now, in a report said to have left Poppi’s mother “profoundly distressed,” the Independent Police Complaints Commission (IPCC) has concluded that senior detectives did not adequately investigate whether Poppi had been abused, despite concerns being raised from the moment she died.

“It is clear from the evidence presented,” said IPCC commissioner Carl Gumsley, “that Cumbria Constabulary’s original inquiry was not fit for purpose.”

The hospital doctor who pronounced Poppi dead on the morning of 12 December 2012 mentioned anal injuries which he stated could not rule out something having been inserted.

At a post-mortem five days later, Home Office pathologist Dr Alison Armour raised concerns that Poppi’s injuries may have been caused by sexual abuse – although another pathologist said they could have been consistent with constipation.

The IPCC accepted that detectives were hampered by a six-month delay in receiving a full post-mortem report, but concluded there was enough evidence to have justified an arrest either on the day of Poppi’s death or shortly after the post-mortem.

In a report, published on Thursday, the IPCC’s lead investigator Tim Kimber stated: “Not only were there suspicious circumstances, there was also a suspect on day one.

“Despite the level of suspicion, and there being a suspect, a crime was not recorded until 28 August 2013, which is the day [the father] was arrested.”

He said there were concerns that Poppi’s father was the last person in her care before her death, and that protecting the girl’s siblings would have satisfied the necessity grounds for arrest.

The IPCC also found evidence that the family home on the morning of the death was not adequately preserved and searched, resulting in a nappy that Poppi had been wearing being lost as potential crucial evidence.

The IPCC said the post-mortem, at which the cause of death was “unascertained”, was the pivotal point of the inquiry. That was when the then lead officer Detective Inspector Amanda Sadler considered the conflicting pathologists’ opinions about possible abuse or constipation.

The IPCC said it was arguable that the result of Ms Sadler not seeking clarification while she was in the room with the two pathologists led to “unclear lines of inquiry”.

When Detective Superintendent Mike Forrester subsequently took on the investigation, he was effectively faced with two possibilities, one of them being the possibility of sex abuse.

But, the IPCC report stated: “There is substantial evidence upon which it could be concluded that these two lines of inquiry were not pursued equally and appropriately and that there was more focus on establishing a natural cause of death.”

Then in March 2014, in a civil fact-finding judgement, High Court family judge Justice Peter Jackson concluded on the balance of probabilities – which is the civil, but not the criminal standard of proof – that the 13-month-old had been sexually assaulted by her father.

The judge stated: “The father is not called upon to prove anything [but] the sequence of events that the father describes is unconvincing.”

He reaffirmed his conclusion in January 2016, despite Mr Worthington continuing to protest his innocence and calling three medical experts who said evidence to support a finding of abuse was lacking.

By this point, though, Cumbria Police had announced – in March 2015 – that Mr Worthington would not face any criminal proceedings. In July 2016, the Crown Prosecution Service also said there was “insufficient evidence” to charge Mr Worthington.

The IPCC report concluded: “There is substantial evidence available to support the contention that this case has still not reached a resolution more than two years on from the death of Poppi because of the unstructured and disorganised approach taken by Det Supt Forrester and DI Sadler, coupled with the argument that prior to the pathologist report being received they did not conduct a criminal investigation despite there being significant suspicious circumstances from the outset.”

Mr Kimber concluded that both Mr Forrester and Ms Sadler had cases to answer for gross misconduct.

Mr Forrester retired so no further action could be taken.

In November 2015 he told the BBC: “I'm not saying we get it right all the time but none of us on this case have done anything intentionally wrong.

“We haven’t done anything dishonest. We’ve done the best we can with the information we had [and] with the resources we had at the time.”

Ms Sadler was subjected to a disciplinary hearing last year where gross incompetency was proven and she was demoted in rank. She too has since retired from the force.

Fiona McGhie, an expert civil liberties lawyer at Irwin Mitchell, speaking on behalf of Poppi’s mother – who cannot be named for legal reasons – said: “Poppi’s mother is understandably deeply concerned by the findings of the IPCC.

“She has anxiously awaited answers as to what happened to Poppi on that fateful day.

“To learn that the actions of senior investigators within Cumbria Constabulary may have contributed to the agonising delays she has endured has left her deeply and profoundly disappointed and distressed.

“She hopes that the long-delayed inquest, now scheduled for May this year, will give her the answers she deserves and allow her to access justice so Poppi can be at peace.”

Responding to the IPCC report, Jerry Graham, the Chief Constable of Cumbria Constabulary, said: “I unreservedly accept the criticisms contained within the report.

“I am absolutely clear that the Constabulary's initial investigation fell well short of the standard that could, and should, have been expected.

“I profoundly regret that we let Poppi and her family down and I offer a heartfelt apology for this.

“Cumbria Constabulary became aware of the deficiencies in the initial investigation in 2014.