The agency has been appointed by Devon County Council, which is determined to stop Exeter City Council gaining the extra powers.
The news comes a week after PRWeek revealed that Weber Shandwick is lobbying for the bid by Shropshire Council. It means that the agency's PA division, headed by Jon McLeod, will have to get behind one bid for unitary status at the same time as criticising another.
Exeter City Council sits within Devon County Council. Exeter claims that unitary status will ensure that money is spent more efficiently.
But Devon argues that such a move will be hugely expensive and will leave less money for people living outside of the city.
Devon's head of comms Peter Doyle confirmed the hire of Weber Shandwick, saying: ‘The brief is to act as professional advocates for Devon County Council in Whitehall and Westminster.'
He added: ‘There are serious flaws in the business case for a unitary authority in Exeter. It is badly thought out and will have dire consequences for the city and the county.'
As revealed last week, Exeter is using Connect Public Affairs to lobby on its behalf. It is one of six councils shortlisted for unitary status to hire a PA agency.
Devon is not the only local authority opposing a bid. Suffolk County Council is critical of Ipswich Borough Council's bid and Norfolk County Council is against the bid by Norwich City Council.