The letter sent to the Architects Newspaper – signed by friends, fellow and former students and employees of Hadid – claims the recent actions of the Zaha Hadid Architects director are the latest in a series of moves that contradict her wishes.
"What is not publicly known is the extent to which he [Schumacher] has been attempting to thwart Zaha's last wishes," reads the letter.
Its signatories are: Mya Manakides, Nicholas Boyarsky, Lisette Khalastchi, Robert Cole, Sand Helsel, Michael Wolfson, David Gommersal, Brian Ma Siy, Alastair Standing, Graham Modlen, Kathleen Peacock, Ban Shubber, Rodney Place and Miska Lovegrove.
"Zaha's unique legacy must be protected"
It was revealed last week that Schumacher had gone to the high court to try get the the other three executors of Hadid's £70 million estate removed.
Hadid had named Schumacher an executor along with her niece Rana Hadid, artist Brian Clarke and former Arts Council chairman Peter Palumbo. The letter pledges support to this trio over Schumacher.
"We wish to express our utmost concern at the recent legal action by Patrik Schumacher against Zaha's fellow executors," reads the letter.
"Zaha's unique legacy must be protected," it continues. "It is of paramount importance that the Zaha Hadid Foundation is established and fully endowed to fulfil the promulgation of her educational vision."
Schumacher's actions "speak for themselves"
Hadid, Clarke and Palumbo have previously been vocal in their criticism of Schumacher, penning an open letter after the architect told the audience at the World Architecture Festival that he believes social housing should be scrapped.
In this new letter, they state that Schumacher's "public behaviour and proclamations... speak for themselves".
Schumacher has already replied to the open letter, which was published on 21 November, by commenting under it from his official Facebook page.
"My dear friends, I wish you had given me a chance to explain to you what is going on before you throw your stones," he said. "Are we no longer on first name terms? Have you seen Zaha's letter of wishes? Let's talk soon."
He added that none of the undersigned are "current employees of ZHA". Earlier in the week he shared an old photo of Hadid and himself sitting together on Facebook.
Control bid claims are misunderstanding says Schumacher
In an earlier statement sent to Dezeen, Schumacher said that claims he was trying to become sole executor of Hadid's estate for personal gain were "falsely reported", and that there had been a "misunderstanding" of his intentions.
"I have been working with Zaha Hadid since 1988 and we have been in close communication every day for 28 years until her passing in 2016. My intentions and actions as executor are faithful to her spirit, hopes and explicit wishes," said Schumacher.
"I applied to the High Court in London for an order to replace my current co-executors with independent professional executors."
The charities regulator has now intervened in the dispute, reported the Times. The Zaha Hadid Foundation, which was founded by Hadid in 2013, has been unable to start new initiatives because the probate process is ongoing.
"We will be engaging with the relevant parties to assess their handling of this matter and to ensure that charitable funds are not at risk," a spokesperson from the charities regulator told the paper.
Portrait of Zaha Hadid is by Mary McCartney.