Both practices, who cannot be exposed for legal reasons, are household names in the world of architecture.
The pair have admitted that they knew that 'brown envelopes' had been handed to Muscovite officials to ease the process of winning planning permission.
The admission comes at a time when the dynamics of conservation and development in the Russian capital have reached a crisis level. Many in Moscow are now claiming that the entire historic fabric of the city is in serious danger of demolition.
The most threatened buildings are those that were built during the Communist era, including some stunning examples of the Modern Movement.
The extent of the uneasy atmosphere in Moscow is illustrated by the fact that the head of Moskomnaslediye - the city government body that looks after historic buildings - now has bodyguards because his predecessor had faced so many death threats.
While the bribing of officials is far from unheard of, it will come as a shock to many that British practices are so brazen about it.
Both offices said that they have not actively handed over bribes, but had been made aware that either planning agents or contractors had produced backhanders.
'I am certainly aware that it has happened on our projects,' one senior architect told the AJ. 'I'm not entirely comfortable about it, but it's just something that happens in Moscow.'
One outcome of the illegal demolition of buildings in Russia in recent years has been the birth of an embryonic conservation movement, often led by foreigners.
Edmund Harris, who helps run the Moscow Architecture Preservation Society, is one such activist.
'In short, brown envelopes are the favoured method,' he told the AJ. 'You sound out the various government officials in the departments that are supposed to look after historic buildings and 'come to an agreement', as the euphemistic Russian phrase has it.
'The actual tactics employed on the ground vary somewhat. One of the most popular ways of disposing of inconveniently placed historic buildings is for documents to be magically produced, saying that the building is a dangerous structure and as such has to be demolished.
'Too many perfectly sound structures have been lost this way. A slightly more subtle way of doing it is for the building suddenly to be wholly or partially 'delisted', so that either the entire building, or else everything apart from the facade, can be demolished with impunity,' Harris added.