A fashion designer died as a result of drug poisoning after taking the drugs GHB and cocaine, an inquest has heard.

Khalid Al Qasimi, 39, the son of the ruler of Sharjah in the United Arab Emirates, was found dead on 1 July by his cleaner at his luxury apartment in west London, where he had consumed the drugs with another man the night before.

Yohan Escobar, whose connection with the prince was not confirmed but who was not treated as a suspect, told police that the GHB was particularly strong, Westminster coroner’s court heard.

The coroner Bernard Richmond QC said a postmortem report by Dr Michael Osborn had confirmed that the cause of death was drug-related.

“I am satisfied that Khalid Al Qasimi, aged 39, met his death when he was in his flat sometime between when he was left by Yohan on 30 June at 9pm and when he was found by his cleaning lady the next day at 10.05am,” Richmond said, according to the Mirror.

“It is plain from what I heard there is no suggestion of any third-party involvement or anything untowards being done by anybody else. It is quite clear from the detective I spoke with that the last person who saw him, Yohan, was spoken to and had explained they had been taking strong GHB and cocaine.

“There was nothing else that would account for his death and he was found in the same place as Yohan had left him.”

He added: “I am quite satisfied on the balance of probabilities that it was a drug-related death.”

Escobar told police he had believed Al Qasimi was asleep when he left the flat, because he was snoring. He was found dead in the same spot the next day.

“He said that the cocaine and GHB in particular had seemed strong,” Richmond told the court, referring to Escobar’s police statement.

“There came a time when Khalid appeared to be on the floor sleeping and the reason why he thought he was sleeping was because he was snoring. And because he was on the floor appearing to be sleeping, Yohan left the premises.

“There is no suggestion of anybody acting in a way to give or make Khalid take these drugs.”

Al Qasimi, the owner of British-based fashion label Qasimi Homme, had taken high levels of GHB and a recreational amount of cocaine, the toxicology report confirmed.

Relatives of the prince were reportedly not present, but his cleaner Maria Kisaohanyan – who had let herself into the apartment and found him sitting on the floor by a sofa, before raising the alarm – paid tribute to him.

“He was full of life, on top of his career and on top of the world, very open-minded and very clever,” she told the court.

Al Qasimi’s older brother, Sheikh Mohammed bin Sultan bin Mohammed Al Qasimi, died from a heroin overdose at the age of 24 at the family’s home in East Grinstead in 1999.