A £1m donation to the National Portrait Gallery has been withdrawn because the potential donors are caught up in the US opioid crisis.

The Sackler Trust, run by the family that owns Purdue Pharma, seller of prescription painkiller OxyContin, said the donation might "deflect" the gallery from its important work.

The Sackler family has "vigorously denied" the allegations against it.

The National Portrait Gallery said it supported the family's decision.

The Sackler Trust offered the money in 2016 to go towards the gallery's £35.5m redevelopment. The gallery had been mulling over whether to accept it.

In a joint statement, the Sackler Trust and the National Portrait Gallery said they had "jointly agreed not to proceed at this time" with the donation.

A spokesman for the Sackler family said: "The giving philosophy of the family has always been to actively support institutions while never getting in the way of their mission. It has become evident that recent reporting of allegations made against Sackler family members may cause this new donation to deflect the National Portrait Gallery from its important work.

"The allegations against family members are vigorously denied, but to avoid being a distraction for the NPG, we have decided not to proceed at this time with the donation."

Purdue Pharma is facing lawsuits in the US alleging that it sold OxyContin as a drug with a low chance of causing addictions, despite knowing this was not true.

The US Centres for Disease Control and Prevention has said that opioids - a class of drug which includes everything from heroin to legal painkillers - were involved in almost 48,000 deaths in 2017.

President Donald Trump has called the US opioid epidemic a "national shame" and declared it a public health emergency.

The epidemic started with legally prescribed painkillers like Percocet and OxyContin. It intensified as these were diverted to the black market. There has also been a sharp rise in the use of illegal opioids including heroin, and many street drugs are laced with powerful opioids like Fentanyl, increasing the risk of overdose.