News of the development follows PRWeek revealing the roster for work worth more than £100,000 had been decided.
The line-up of Engine Group, Munro & Forster, Consolidated, SKV Communications, Unity and Syndicate Communications will now compete for work.
However, one well-placed PR source noted that some big-name agency omissions had led to moves to try to sidestep the new set-up.
'At least one department is looking into how it can get around this and continue to pick agencies itself,' he said.
The new roster forms part of a major updating of government comms, which included the closure of the COI last year amid marketing spend cuts of £390m. It was built by the Government Procurement Service.
The new, more centralised approach across marcoms includes the Agile Route to Market framework, which covers PR work under £100,000.
A Cabinet Office spokesman said: 'This framework means we can spend money in a more strategic and co-ordinated way, with confidence we are extracting maximum value.'
The Cabinet Office website states 'all in-scope spend' should be channelled through the framework. This is understood to include departmental work, 'arm's-length bodies, non-departmental public bodies and agencies', but not the NHS or local government.
Among the large agencies that have worked for the Government but did not make the cut are Blue Rubicon, Weber Shandwick, Grayling and Freud Communications. Freud's £1m-a-year brief with the Department of Health is unaffected.
PRWeek understands discussions have taken place between senior figures at some of the omitted agencies regarding a possible objection. One claimed the fact that the roster selection process judged 40 per cent of marks on the basis of price was indicative of a decision based on 'the cost of everything and the value of nothing'.
Oliver Hickson, director of Hickson Communications and former head of PR at the COI, called the decision to exclude such big names 'a risk' and added departments would 'find a way to do their own thing'.
Appointed agencies were unable to comment due to an EU-enforced ten-day 'standstill period'.
Ex-government comms secretary says agencies will want to know they are taken seriously.
The Government is now going to have to prove to agencies that it 'takes comms seriously', it has been claimed.
Former permanent secretary for government comms Matt Tee has said there is still a lot of confusion around the new set-up.
'There's been a fair amount of unhappiness in the industry about the process and how long it's taken, and agencies will be looking to see that Government takes comms seriously,' he said.
PRWeek revealed last year that the roster bidding process had been beset by delays.
Tee said: 'The Government needs to think about how it's going to approach working with creative agencies, as at the moment its reputation as a client is not great.
'People understand why the COI was abolished, but they understood how it worked and many don't yet understand the new arrangements.'
Tee's words follow claims in PRWeek's sister magazine Marketing that the Government Procurement Service had created 'ruptures' in the process of selecting the advertising roster, which has just been announced.
Tee, who left his government position in 2011 to launch his own consultancy Reputate, will take a leading comms role at the NHS shortly.
How I see it
Alison Clarke, CEO, Grayling
Price has been a deciding factor for the Government and I wish those agencies who have been successful very well indeed. Clients need to have an appropriate choice of agencies that can offer high quality support and consultants who have a deep understanding of their specific sector at an acceptable price.
Laura Oliphant, MD, Stand Agency
I like the fact that the roster wasn't predictable. They seem to have picked the top six responses rather than the top six names and that means you will get new talent and experience coming to the table.
78 Number of PR agencies on the Agile Route to Market roster*
6 Number of PR agencies on the new roster for briefs worth £100,000+**
£390m Government marketing spending cuts ahead of the COI's closure*
£285m The Government's planned comms spend for 2012-13***
Source *Cabinet Office; ** Government letter sent to agencies; ***Number 10.