Almost exactly 25 years after Prince Charles began his personal battle against modern architecture, condemning a planned - and swiftly abandoned - new wing to the National Gallery as a "monstrous carbuncle", he has received his sternest ticking-off yet from the profession, with some of the world's leading architects warning that he is abusing his position to unfairly influence planning decisions.

The prince's private efforts to scupper a development of glass and steel housing blocks by Richard Rogers' practice at the site of the former Chelsea Barracks in west London amounts to an attempt to "skew the course" of an open planning process already under way, the architects, including Lord Norman Foster and Zaha Hadid from the UK, as well as Frank Gehry, who created Bilbao's Guggenheim museum, wrote in a letter.

Such interference undermined developers' confidence in the UK's planning system and might even deter some from investing in London, one of the other signatories, a leading urban planning expert, Professor Richard Burdett, warned.

"We have here a very established, very clear, open and democratic process, with all its strengths and weaknesses," said Burdett, who headed the Venice architecture biennale on cities in 2006. "We know how it works. It has the possibility of an appeal and then the possibility of a public inquiry. The fact that suddenly a voice which is somehow unequal comes into it does create a risk of destabilisation.

"In this delicate moment of post-credit crunch economic frailty, developers could feel that their money is at risk, or made more at risk, by powers that are not very clear. We could end up in a situation where people end up saying, 'Why invest here?'"

The 10 senders of the letter to yesterday's Sunday Times, among them six winners of the Pritzker award, often viewed as architecture's equivalent of a Nobel prize, were prompted to act after it was reported that Charles had written privately to the head of a firm owned by Qatar's royal family which bought the 5.2-hectare site for more than £950m last year.

It is understood that the prince called Rogers' proposed flats, to be located opposite Sir Christopher Wren's Royal Hospital, "unsympathetic". Charles is believed to favour an alternative design by the architect Quinlan Terry, which mimics Wren's building and has been condemned by critics as a weak pastiche. Backers of the Rogers design also point out that about half the 552 units in his scheme would comprise affordable housing.

Charles's "private comments and behind-the-scenes lobbying" were anomalous in a modern, democratic system, said the letter writers, who also included Jacques Herzog and Pierre de Meuron, the Swiss duo whose practice turned the former Bankside power station into the Tate Modern, and Italy's Renzo Piano, the co-designer with Rogers of the Pompidou Centre in Paris.

Rogers' proposals for the Chelsea site had already been adapted following local objections and were now in the hands of Westminster council planners, they noted. "If the prince wants to comment on the design of this, or any other, project, we urge him to do so through the established planning consultation process. Rather than use his privileged position to intervene in one of the most significant residential projects likely to be built in London in the next five years, he should engage in an open and transparent debate."

"This is not really about a style, or an argument about how buildings look, but how we go about things," said Deyan Sudjic, the director of the Design Museum in London, who also signed the letter. "What's slightly depressing is that this is kind of an old argument which began 25 years ago in quite a similar way, with the Prince of Wales' no doubt quite well-intentioned attempt to interfere with a process which does have certain clearly laid-out legal steps. This is an unaccountable additional layer to that process."

While Charles's views on modern architecture have remained trenchant and regularly aired, in recent years he has repaired some bridges with the profession. Next month, he is due to present a lecture marking the 175th anniversary of the Royal Institute of British Architects.

"I think it's all come as a bit of a surprise," said Sudjic. "With the Prince of Wales due to be addressing Riba shortly, there was a sense that sweetness and light had broken out and that there wasn't quite the stylistic polarisation that there had been. Now it seems to be the prince, again, targeting a particular architect in a way which is returning to the way that he behaved previously."

A day after Riba announced that Charles was to make his address, the organisation's president, Sunand Prasad, criticised the prince's intervention, saying he should "allow the properly constituted and conducted planning process to take place unhindered".

A spokeswoman for the prince's Clarence House office said she had no comment about "a private letter that may or may not have been sent".

Charles v the architects

Prince Charles's views on modern architecture are well known. In May 1984, he used a speech marking the 150th anniversary of Riba to lambast the National Gallery's planned extension. The design was dropped. Charles also agitated successfully against a modern scheme devised by Rogers for Paternoster Square adjoining St Paul's Cathedral in London. The prince has for some years largely avoided criticising specific buildings still at the planning stage, although this rarely stopped him airing displeasure at existing buildings or the modern architectural establishment in general - in February last year he labelled a highly praised new university lecture hall in Essex "a dustbin". The prince's own foray into urban design, Poundbury in Dorset, has been received with equal sniffiness by many architects. The village's new mock-Classical fire station was described by one critic as "the Parthenon meets Brookside". The paradox of his latest intervention is that when Wren built the Royal Hospital in the 1680s it was modelled on the then new Hôpital des Invalides in Paris, and was thus deemed modern itself.