Speaking at the London hustings last night, Phillips was forced into admitting his membership of the extreme right-wing party for fear of being exposed by a journalist.
The national councillor, who is standing against Valerie Owen and Sunand Prasad, makes no mention of his political allegiances in his manifesto or his campaign website.
The news has triggered an outcry in the architectural community. Society of Black Architects chairman Chris Nasah: 'I call on people not to vote for him. I will say that categorically.
'Obviously people have a right to their own political opinions. But we need someone who can build bridges between communities and not someone who wants to break them down.'
Unsurprisingly others have echoed Nasah's comments. Past president George Ferguson said the news 'deeply saddened' him.
'It is saddening that there are people out there in the profession that believe in this kind of thing,' he said.
'Rather than call on people not to vote for him, I'd rather point out that there are two other exceptional candidates to support.'
One RIBA organiser in the North of England, Jane Simpson, claimed that members in the area had to be careful to make the information public. 'We need to do everything we can to ensure that he's not elected,' she said.
Prasad was careful not to turn the election debate personal. 'I do not want to make a huge fuss about this,' he said. 'But the BNP tries to present itself as a reasonable organisation.
'It is, however, really about racial purity. It makes claims such as white people are genetically more predisposed to democracy and the rule of law.
'People need to be aware of this. The trouble is that the BNP is a leech on the very real and reasonable fears of the people that have been let down.
'Peter is free to stand though, and I am pleased he's being open about it,' he added. 'I think the vast majority of people will see what's going on and will see him for who he is.'