Boris Johnson appointed a key figure to the Grenfell Tower inquiry who has links to the company which made the cladding blamed for accelerating the fatal fire, the Guardian can reveal.

Last month, the prime minister picked Benita Mehra, an engineer, to assist Sir Martin Moore-Bick, a retired judge who is leading the inquiry into the disaster that claimed 72 lives. Mehra previously ran an organisation that received a £71,000 grant from the charitable arm of Arconic, the US conglomerate that made the aluminium composite cladding panels used on Grenfell.

The inquiry has already found that Arconic’s polyethelyne-filled panels were “the principal reason why the flames spread so rapidly up the building”. The Arconic Foundation’s board of directors includes several senior Arconic executives and its stated goal is to support the company’s mission by making grants in countries where it trades.

Survivors and the bereaved said the grant created a clear conflict of interest and described Mehra’s appointment as “a slap in the face” for their hopes of justice. Grenfell United is calling for Mehra to stand down before hearings restart on 27 January with an examination of how the Arconic cladding panels were chosen, their safety testing, marketing and promotion. Mehra is one of two experts selected to help Moore-Bick preside over at least 18 months of hearings into the events leading up to the fire.

“How can she sit next to Sir Martin Moore-Bick when Arconic will be on the stand and is one of the organisations we need answers from in terms of what caused the deaths of our loved ones?” asked Karim Mussilhy, the vice-chair of the survivors and bereaved group GU. “Her society has been supported by Arconic. She will look at it from the perspective of Arconic doing good things for the industry, that they are a great organisation. Her perspective will be affected.”

The Arconic Foundation grant was awarded to the Women’s Engineering Society charity, which Mehra chaired from 2015 and 2018. She helped draft the application and the funds arrived three months after Grenfell in 2017, the charity confirmed. It was the largest grant received that year. Mehra remains a trustee.

The link is particularly sensitive because anger is running high among many survivors at the role played by manufacturers of the combustible cladding and insulation materials used to reclad the tower during its 2016 refurbishment. The families of 69 victims and 177 survivors are separately suing Arconic and other materials manufacturers in the US courts for wrongful death. It has argued any litigation should take place in the UK. The UK government has also banned the use of such panels on high-rise residential buildings.

A spokesperson for the inquiry said it was “confident that Benita Mehra’s former presidency of the Women’s Engineering Society does not affect her impartiality as a panel member”.

A Cabinet Office spokesperson said there were “robust processes … [to ensure] any potential conflicts of interest are properly considered and managed”.

“The Arconic Foundation donated to a specific scheme which provides mentoring for women in engineering and is unrelated to the issues being considered by the inquiry,” they said.

The inquiry concluded in October that Arconic’s panels were the main cause of the spread of fire that engulfed the full height of the 24-storey tower in less that 30 minutes on 14 June 2017. Moore-Bick said the panels “melted and acted as a source of fuel for the growing fire”.

“We will be absolutely furious if she is on the platform and it would be morally wrong to keep this person there,” said Mussilhy. “The report from the first phase of the inquiry restored a little bit of confidence. This has taken us 10 steps backwards.”

Mehra’s appointment was quietly announced by Downing Street just before Christmas on 23 December, in a move suspected by some of the bereaved and survivors to have been an attempt to avoid public scrutiny and possible legal challenge. Johnson said at the time that Cabinet Office officials had conducted due diligence about Mehra’s appointment, “which has not identified any concerns”.

He told Moore-Bick in a letter: “The [Inquiries] Act is also clear that I must not appoint … a person who has a close association with an interested party unless those links could not reasonably be regarded as affecting the impartiality of the inquiry panel.”

He said: “Ms Mehra has confirmed that she is not aware of any conflict of interest.”

Mehra was a late replacement for Prof Nabeel Hamdi, an expert in housing and planning issues who survivors hoped could better understand the tensions between the council landlord and the tenants that preceded the disaster than Moore-Bick, whose expertise is in construction contracts. Johnson provided no explanation for the move apart from saying Hamdi “was unable to proceed with the appointment”. The Cabinet Office said this week that Hamdi had reflected on the commitment required and decided to withdraw.

Concerned that Mehra lacked his expertise in community relations, Grenfell United decided to investigate her background and found details of the Arconic Foundation’s grant in the annual report and accounts of the Women’s Engineering Society.

The inquiry panel was only introduced by Theresa May after months of pressure from survivors backed by the musician Stormzy to appoint people who understand “the culture at the heart of how people living in social housing are treated”.

A spokesperson for Arconic said the foundation was an independently endowed and managed foundation, with a core goal to advance science, technology, engineering, and mathematics education and training worldwide and create access to these fields for girls and women internationally.

“The grant we awarded in 2017 to this particular UK association was purely on this basis,” they said.