By Ben Davies
BBC News politics reporter
Brendan Barber has warned the leaders of three key UK unions not to turn their backs on the TUC if they decide to amalgamate into a 'superunion'.
This is Barber's third annual congress as TUC general secretary
The TUC leader said he saw benefits from the T&G, GMB and Amicus combining but said his organisation was still best placed to speak for all workers.
If the three were to join forces they would have a membership 2.6 million.
Mr Barber said it would be "corrosive" if they gave an impression of indifference to the wider movement.
There have been reports that the annual fee to the TUC of such a superunion would amount to £5m and that questions are being asked as to just what members will get in return for such a substantial outlay.
Mr Barber said: "I think the voice of trade unionism will be at its most influential on government and anywhere else for that matter if it's seen to be a united voice.
"And the TUC is the mechanism that brings unions together reflecting diversity of workers right across the economy and provides a forum in which we can forge that common view."
Mr Barber acknowledged Amicus, the T&G and GMB already worked alongside each other in many companies and there was "plenty of scope for minimising the duplication that takes place".
But he warned each of the three unions not to take their eyes off issues such as recruitment in favour of the "inevitable internal debate" about new rules and constitutions.
"I am very conscious that mergers of themselves don't make a single extra trade union member," he said.
"I also think it's very important that they don't allow the impression to be created that they are somehow indifferent to the rest of the trade union movement or that they don't recognise the importance of the work other trade unions do and the expertise they bring to bear in the areas in which they operate."
Mr Barber added that such an amalgamation would "change the landscape" of British trade unionism "but I would certainly hope that it wouldn't be at the expense of relations with other unions and the crucial role of the TUC as a single coherent union voice".
And he insisted that there was "no general mood" for leaving the TUC within the T&G, Amicus and GMB.
"Most people around the trade union movement do attach importance to having a strong, central, national TUC and I am confident that that's the view that will prevail," said Mr Barber.
He said the TUC had a role to play in ensuring any new union did not "go it alone" and continued to work with other unions in "a proper collegiate way".
Mr Barber insisted that splitting with the TUC was not something that had been "coming across sharply" but there had been "occasional suggestions about the new union being so powerful it would have not need to talk to anyone else".
He added: "Those messages would be very corrosive in trade union relationships right across the field."
The TUC general secretary was speaking as a report from London School of Economics (LSE) Centre for Economic Performance was published suggesting only 12% of privately employed workers are union members and total union membership had fallen by five million since 1979.
Research by the LSE also indicated the proportion of the workforce that belongs to a trade union is lower than when Labour came to power.
Mr Barber acknowledged the union presence had to grow in the private sector and said some of the major unions had already begun to focus their energies and resources "in a different way" to recruit more members.
TUC delegates begin gathering in Brighton over the weekend for the annual congress.
One of the key issues on the agenda is the looming pensions crisis, with government pensions expert Adair Turner due to address the conference.
Union representatives have been engaged in talks with ministers over controversial reform plans including proposals to up the retirement age for public sector workers from 60 to 65.
Strike action over public sector pensions was narrowly averted just before the last election.