Unions need to join forces with their counterparts in the rest of Europe to lead the fight against multinationals, according to the head of the biggest union involved in merger talks.

Derek Simpson, general secretary of Amicus, the largest of the three unions involved in talks to form a super union in Britain, also hinted that membership risked becoming a sham for many workers unless the unions managed to boost their position against international companies.

"We need to develop the principle of a European trade union," said Mr Simpson. "Joint action with colleagues on the continent would stand a better chance of affecting a company's plans."

The comments, made in an interview with the Financial Times, suggest that the face of Britain's union movement could change radically during the next few years. It also underlines the crisis the unions are facing, with the decline in both membership and bargaining power.

If the proposed merger of the GMB, the Transport and General Workers' Union and Amicus goes ahead, the resulting entity would account for more than 2m fully paid-up members. This, Mr Simpson said, could help it to fight "the two biggest concerns that people have in employment today: job security and pensions".

Although Mr Simpson conceded that a super union in itself would not necessarily raise bargaining power against individual employers, he said the merger could help unions co-operate when a multinational was trying to play one country's workforce off against another by threatening to transfer jobs abroad.

It would also create a united force that would carry more weight with the Labour party, enabling them to push for national employment rights that prevented multinationals from shutting British factories where workers were "quicker, cheaper and easier to dismiss".

But he did not want to use a super union's financial clout as a party donor to blackmail the government.

"You can't sit around saying, 'we're not going to give you the money to win an election unless you do this, that or the other', because it's not in our interest to end up with a Tory government."

Mr Simpson was brutally honest about the British unions' influence in Europe. At the moment they were "small players" compared with Germany's behemoth service sector unions, IG Metall or Verdi. "Why would Jurgen Peters of IG Metall bother talking to me, if I go to him saying, 'look, Sie mens has shut its plant and it's gone to Germany?'"

However, even if there were no pan-European mergers, the British super union would have more chance of making unions on the Continent listen and fight for the common interest.

He said the greater co-operation of Britain's union movement had al- ready borne fruit, with last year's Warwick agreement on workers' rights between ministers and the unions.

"By the evidence of Warwick, we are capable of producing a better result together then separately. A merger of three of the big four would be a consolidation of that position."

Mr Simpson, 60, was frank about why the merger was so important. "Back in the days when I was an apprentice we used to call them international capitalists: big corporations that had the ability to move around the globe almost at will. And we could no more affect their actions then than perhaps we can now."

At that time, they employed perhaps 10 per cent of the union's potential membership, "and for 10 per cent of our members we may have been a bit of a sham because we could not really affect the actions of global companies".

"What's happened now is that it's not 10 per cent any more. In fact, it's a majority of companies."

The unions' struggle to maintain power, as business became more international and labour markets in different countries competed, was reflected in the declining union wage premium.

The extra amount a private sector union member earned above a non-union member with the same skills fell from about 10 per cent in 1993 to just above zero by the end of the decade, according to one estimate.

The unions have a generally poor record in preventing job losses in declining sectors such as manufacturing, though with notable exceptions such as the resurrection of Rover.

They also have a mixed record in battling over individual employers' pension schemes. Although there had been notable successes since the ISTC union fought and won the first industrial dispute in defence of a final salary scheme in 2002, the total number of final salary schemes had fallen sharply during the past few years.

Mr Simpson said the potential merger of the unions, which still faced many hurdles, was not likely to boost wages. "If the employer's in a good profitable position and we've got good negotiators it will probably produce more or less the same result . . Thepurse hasn't got any bigger or smaller," he added.