Two takeaway workers have been found guilty of manslaughter after a teenager with a nut allergy died after eating one of their meals.

Megan Lee, who was diagnosed with a peanut allergy at the age of eight, had an asthma attack a few hours after she and a friend ordered food online from a takeaway in Lancashire.

Lee, 15, had the allergic reaction after ordering from the Royal Spice Takeaway in Oswaldtwistle, Lancashire, on 30 December 2016.

After consuming the seekh kebab and naan, she had an immediate reaction and went on to suffer irreversible brain damage. She was pronounced dead on New Year’s Day, 2017.

On Friday, Mohammed Abdul Kuddus, 40, the takeaway’s owner, and Harun Rashid, 38, alleged to be the manager, were found guilty of manslaughter by gross negligence. The pair had denied the charges.

Rashid, of Haslingden, who had claimed he was a delivery driver at the restaurant, was also found guilty of failing to discharge a general duty of employers and another count of failing to put in place permanent procedures relating to food safety regulations.

Kuddus, of Blackburn, had already pled guilty to those two charges on behalf of himself and on behalf of Royal Spice Takeaway Ltd.

Lee’s family members were in tears in the public gallery as the two men were found guilty.

Her friend wrote “prawns, nuts” in the notes section of the online order form for food which did not usually contain either ingredient, Manchester crown court was told.

Peter Wright QC, prosecuting, said staff paid no attention to the entry and served a meal including an onion bhaji, a peshwari naan and a seekh kebab, which tests later showed had the “widespread presence” of peanut protein.

The online menu did not include a list of ingredients for the order and simply bore the rider “Think Allergy” and “please ask any member of staff”, he said.

Harun Rashid and Mohammed Kuddus arrive at Manchester crown court, where they were found guilty of manslaughter. Photograph: Danny Lawson/PA

After the takeaway was delivered to her friend’s home at about 6pm, the girls shared the food but Megan suffered an “immediate reaction” when she began to eat the kebab.

Her friend noted Megan appeared “lumpy” and the girl’s mother gave Lee some liquid antihistamine which made her feel better.

Lee avoided the seekh kebab as she continued eating and, except for a rash on her left cheek, she showed no signs of discomfort when her mother, Gemma, collected her.

Wright said Megan went upstairs to her bedroom but shortly after her mother heard her daughter call out.

He said: “Megan was in a state of distress. Her lips were swollen and blue, she was struggling to breathe and an ambulance was called. Megan’s condition continued to deteriorate. She stopped breathing and her heart stopped.”

Gemma Lee and paramedics tried in vain to revive Megan but she had suffered irreversible brain damage and was pronounced dead at hospital on the morning of 1 January when her life support was switched off, said the prosecutor.

A police investigation was launched. Local trading standards and environmental health officers also inspected the takeaway on 6 January and immediately closed it.

Wright said: “It soon became apparent there were no procedures in place in relation to allergen management and no audit of their available dishes or written records of their recipes was either made or kept.

“The premises were not clean. There was evidence of mouse droppings, dirty work surfaces and pans piled up and left unclean.”

Samples of foodstuffs prepared on the premises were tested and detected the “widespread presence of peanut protein of levels that were unsafe for people allergic to peanuts”, he added.

Wright said the safety inspection also revealed there were no systems for cleaning or to avoid contamination or cross-contamination of ingredients and dishes.

He said Lee, who lived with her younger brother and parents, was studying for her mock GSCE examinations.

As a child she was diagnosed as asthmatic with symptoms controlled by preventative medicine and an inhaler, and at the age of eight was found to be suffering from an allergy to peanuts and “various common inherent allergies”.

Lee and her family understood those allergies to be “mild” and they generally avoided processed food, Wright said.

He explained that a copy of the order form submitted by Lee’s friend was printed off at the Royal Spice and food was prepared on-site before dispatch.

He told jurors: “The entry could have been more specific you may feel but we say the import of the entry was obvious in ordering dishes that did not ordinarily contain either such ingredients and was designed to alert the staff at the takeaway to the risk such foodstuffs pose to a potential customer.

“No attention was paid to the entry by anyone at the takeaway, or if it was, too little to have been of any consequence. If it had been, we say the least that may have happened was that the meaning of the entry would have been explored.”

Wright told the court Lee’s death was a disaster waiting to happen.

The takeaway has since reopened under new ownership. The two men were released on bail to be sentenced on 7 November.