Prince Charles wrote a letter complaining about the scheme
London's Chelsea Barracks developers have made a £81m claim at the High Court, blaming Prince Charles for the withdrawal of a planning application.
The Candy brothers say their Qatari partners were swayed after the prince complained about the scheme for the UK's most expensive housing project.
The court heard the prince wrote a letter saying his "heart sank" when he saw architect Lord Rogers' designs.
The plans were to build 552 modernist flats in glass and steel on the site.
Lord Grabiner QC, representing the brothers' company CPC Group, told Mr Justice Vos that the Prince of Wales had written to the Qatari prime minister, who is also chairman of the Qatari Diar development company.
"He urged Sheik Hamad bin Jasim to reconsider the plan before it was too late and attached a scheme by a different, classical architect he preferred," he said.
Lord Rogers designed the complex that was to replace Chelsea Barracks
"Prince Charles and Lord Rogers had form in the way that they had previously crossed swords and Prince Charles's opposition to modern architecture is notorious."
Nick and Christian Candy are claiming the Qatari company breached the terms of their contract and must now make the payment that was due when Lord Rogers' scheme won the approval of planners.
Lord Grabiner said Qatari Diar could have mounted a "robust defence" of the plan and said that, notwithstanding Charles's intervention, it was going ahead.
Qatari Diar could also have considered the request from Charles. But from March to April 2009 all the company did was to "flounder" and was not prepared to take on Charles publicly without the "say so of the Qatari royals", Lord Grabiner added.
The 12.8 acre site, in one of London's most expensive residential areas, was sold by the Ministry of Defence for £959m to Qatari Diar and CPC Group.