The ARB Reform Group members suddenly departed, citing a failure of the board's leadership to negotiate over several issues.
Some believe the move left the board inquorate - as four of the seven elected members must be present for business to be conducted - but this is unclear currently.
The ARB Reform Group was elected, with the support of the likes of Richard Rogers and Norman Foster, earlier this year with a manifesto to pare back the work of the board.
The five, who all won more votes than the other two elected architect-members, believe in a minimalist reading of the Architecture Act, which set up the ARB in 1997.
The potential consequences of the walk-out remain uncertain, although the threat to carry out the action again will leave the board on the brink of meltdown.
The three issues up for debate that pushed the reformers to act were over the ARB's policy on professional standards, the prosecution of non-architects trading as architects, and the make-up of the board's policy committees.
On the last matter, the group was infuriated when a motion to increase the number of architects on each committee to the same number as the lay members was rejected.
In the words of one board-member, 'the group felt that they had tried to be reasonable and to negotiate, but they made no progress. They had clearly realised there was no more they could do'.
In a rare public statement, chairman Humphrey Lloyd hit out at the five reformers saying he and the remaining board members had been 'surprised and disappointed' by their actions.
'Membership of the ARB- obliges a member to provide the board with the experience and the knowledge for which that member was elected or appointed,' Lloyd said.
'I hope that those who left the meeting will recognise that obligation and will return to play a full part in order to serve, in the public interest, the profession that elected them,' the former judge added.
But RIBA president Jack Pringle, a long-term critic of current ARB policy, said things were coming to a head.
'While I'm not privy to what went on in the meeting, if what seems to have happened, has happened, then it is evidence that we do not have a proper relationship between architects and the board,' he said.
'We at the RIBA want to have a board that we have confidence in,' he went on. 'Until we reform the ARB it will be like a weeping sore. We need to get it sorted out - we all have better things to do.'