The Communication Group found ‘significant concern' about future access to ministers when it questioned 100 PA directors at major UK companies.
Almost 40 per cent said a Brown-run government would be less likely than Tony Blair's administration to listen to lobbyists.
Respondents, surveyed in the past three weeks, said a Brown administration would be ‘run by an even smaller group', be far ‘more difficult to engage', ‘more suspicious of corporate agendas' and ‘less willing to listen'.
Greater regulation and interference in business at the hands of government were among lobbyists' biggest fears.
Elizabeth Truss, TCG public affairs MD, said: ‘Overall there was an expression of significant concern. Public affairs directors will need to have well-prepared strategies to influence government and not just rely on contacts.'
However, one anonymous survey respondent said: ‘Though Brown will be more difficult to lobby, in the next Labour government there will be just one centre of power rather than two.'
Ed Owen, executive director of Euro RSCG Apex Communications, told PRWeek that the landscape would be simply ‘different', rather than more difficult.
‘There will be a different philosophy, a different coterie of people coming in to whom the [PA] industry is yet to be exposed,' said Tim Fallon, head of the corporate practice at Hill & Knowlton.
Lobbyists seeking clues to a Brown administration's personnel should investigate The Smith Institute - the think-tank set up in memory of former Labour leader John Smith - said Fleishman-Hillard associate director Gavin Megaw. ‘It is likely that a lot of staff will move [to a Brown government],' he said.