Unison general secretary Dave Prentis was speaking exclusively to PRWeek.
He spoke out as Unison released figures that show social work vacancies are standing at 'danger level', with 12 per cent of posts vacant across the UK.
Prentis said there was a 'huge job' to be done in promoting the social services as a profession.
He added: 'It would make a big difference if social workers were able to explain what their job involves. Unfortunately, most councils will gag social workers and stop them from speaking out. When you are trying to explain what social workers do, giving them a human face is important, but that is hugely lacking in social services.
'Anyone who works in social services knows they are responsible for saving a lot of lives and improving the quality of many people's lives, but that is not what is being reported.'
Prentis also pointed out that when the media came to Unison for comment from a social worker, the speaker usually ended up being presented anonymously with blacked-out faces and changed voices.
A spokesperson for the Local Government Association (LGA) responded: 'It may have been historically the case that there was some caution, but we do not believe that is the case any more. We are arguing that we need to help social workers talk about their jobs as much as possible.'
The LGA pointed to recent coverage such as BBC One's The One Show's feature on Cheshire West and Chester's social services, and coverage of London social workers in The Guardian.
Brighton & Hove City Council head of corporate communications John Shewell said: 'I do not know where Unison got its information about councils gagging their social workers from speaking to the media, but if this is true then I would urge these councils to review their approach.'
Unison, which represents 40,000 social workers, stated on Monday that morale had hit 'rock bottom' in social services'.
The union has drawn up a ten-point plan for change in the social services, which includes an 'urgent action plan to fill vacancies and to review staffing levels across all social work teams'.
It also calls for measures to 'rebuild morale, confidence and status of social workers'.
HOW I SEE IT - SIMON JONES, Head of comms, London Borough of Hammersmith & Fulham
The negative agenda against social workers is a national issue and requires a national response to encourage people to enter the profession based on the job's everyday rewards and success.
Local positive case studies play a part, but individual local authorities cannot tackle this alone. One of the reasons why we are in this mess is because local authorities have had to go it alone in dealing with a complicated set of media issues that occur when cases go wrong.
It is up to the PR profession to ensure we are sharing best practice and knowledge.