A forensics backlog is said to be holding up the investigation into Erin Patterson’s fatal meal (Picture: Channel Nine)

Detectives might need several more months to solve the mysterious deaths linked to Erin Patterson’s infamous mushroom lunch, it has been claimed.

There is still no official narrative on the case nearly a month after three of the Australian mum’s guests, including her in-laws, died, while a fourth survived a gruesome illness.

Ms Patterson, who survived despite claiming she ate the same beef Wellington thought to contain deadly mushrooms, remains under no suspicion.

Police have not issued an update on their investigation since more than two weeks ago, when they stressed they were still working with an ‘open mind’ over whether there was any deliberate attempt to harm the diners.

According to Daily Mail Australia, the forensics team looking into the case have been asked to prioritise its analysis of biological materials linked to the case, which has gripped the world.

Staff were said to be so overworked that cases have been pushed back by up to a year as a result of forensics backlogs.

The newspaper cites police sources saying that detectives on the case will hardly have their ‘cue in the rack’.

Detectives are keen for tests of biological materials to come back (Picture: AAP Image)

The materials which need testing are likely to include samples taken from leftovers of the pastry dish which Ms Patterson, a 48-year-old amateur cook from Leongatha, Victoria, handed over to local police.

They may also include any quantities of fungi found around her house, which experts could try to compare to samples found in the food.

Ms Patterson has previously claimed she used two types of mushroom – a dried variety bought months earlier and a fresh variety bought recently from a local supermarket – though it’s not clear whether these were seized in any searches.

According to a friend of the family, she is known to be ‘very good at foraging’, picking and identifying wild mushrooms in the surrounding countryside on a regular basis.

Don and Gail Patterson died days after eating Ms Patterson’s meal (Picture: Yvette Kelly)

The presence of mushrooms in the dish is not thought to be of any controversy, as beef Wellington recipes classically call for a layer of finely minced mushroom between the meat and pastry casing.

At the last press briefing on the case, Detective Inspector Dean Thomas initially said the deaths were ‘not suspicious’ before appearing to suggest Ms Patterson ‘was and is’ a suspect ‘because she cooked those meals’.

She was released from custody following a police interview the same evening.

DI Thomas added at the time: ‘We’ve still got a lot of work to do. We need to understand what’s caused these symptoms, what’s caused ultimately the deaths of these three people.

Gail’s sister, Heather Wilkinson, died, while her husband Ian survived

‘We need to understand what has occurred, what is the cause of the injuries, what has caused their deaths and the circumstances surrounding all of that.

‘So it’s a very complex matter. It’s really interesting. Four people turn up and three of them have passed away.’

It’s thought his team is also eagerly awaiting results of the toxicology tests carried out on the victims.

Gail and Don Patterson, who were the 70-year-old parents of Ms Patterson’s estranged husband, and Gail’s sister Heather Wilkinson, 66, all died within days of the meal.

Ms Wilkinson’s husband Ian, 68, was also at the lunch and miraculously survived after spending days in a coma on life support in hospital.

Their symptoms were said to be consistent with the consuming of death cap mushrooms, which cause catastrophic damage to the liver.

Ms Patterson denies any wrongdoing and says she did not know the mushrooms were poisonous.

Delays in testing biological samples linked to Erin Patterson’s deadly meal are thought to be hampering Australian police.