In an exclusive interview with PRWeek, the ex-BBC correspondent, who left the transport department after falling out with ex-minister Stephen Byers and former adviser Jo Moore, attacked the government for attempting to block his efforts at making the programme for Channel 4.
'The government has been incredibly obstructive,' he said. 'They have issued blanket instructions to ministries not to co-operate with it.'
Sixsmith claims that his request to film at the Civil Service College for the documentary has been denied.
The Cabinet Office rebutted the claim, saying: 'We did turn down requests for a private meeting. We always turn down requests for private meetings.'
Sixsmith's central charge, that the government is refusing to co-operate with the production team, was denied by a Cabinet Office spokesman, who said: 'We have assisted the production company in their enquiries to date and are still in communication with them.'
The spokesman also denied there were any 'blanket instructions' issued to government departments.
Sixsmith left the DTLR last month after a spat in which his resignation was announced by Byers before he had agreed to resign. His current TV project is expected to be aired in September, during the political conference season.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.