Tony Blair has urged trade unions to modernise in order to seize the opportunities of globalisation and the changing nature of work.
Unions could offer 'tremendous' benefits to workers, Mr Blair said
Unions could become "a revitalised part of British society", the prime minister told a London conference organised by think tank Unions 21.
But that would require "profound organisational change", he said.
"The world has changed. The role of trade unions must change with it," he told delegates from over 40 unions.
Mr Blair called on unions "to advance the interests of people in a modern and insecure workplace, with a far broader range of services and support than is traditional".
He told delegates: "If they embrace social partnership, if they assert a true and necessary role in community action, trade unions could become a revitalised part of British society with tremendous and beneficial consequences for people at work and for the community."
He added: "There is a huge opportunity today for modern trade unionism. Seize it. There is enormous potential in what trade unions can offer."
The prime minister said trade unions' influence would be "in proportion to the weight they have in broader society and the reasonableness of the case they make".
"The reality is that the only political power that can be exercised by unions today is of the small 'p' variety.
"It derives from their standing and support among people. In turn, this derives not from industrial militancy or historical party relationships, but from how well unions perform their proper functions."
TUC General Secretary Brendan Barber told the conference that unions had a "huge agenda to work on".
"This audience knows well enough the challenges that unions face," Mr Barber said.
"The challenge of increasing our membership, of breaking through to build organisation in the growing services sectors.
"A challenge to change and adapt as the world of work changes around us, and the challenge of winning a greater recognition as the authoritative voice of Britain at work."
He added: "Our destiny ultimately is in our own hands, but if we are able to shape a more positive relationship with government then our opportunities will be that much greater."
Mr Barber said the government had to stand up to employers over pensions.
"If they don't, I think their standing in the trade union movement will be hugely damaged, the crisis in private sector pensions will grow deeper and any short-lived gratitude from Britain's boardrooms will not make up for the inevitable reaction from voters."
He said the government should adopt measures recommended by the Pensions Commission, including compulsory contributions by employers.