The head of the country's biggest manufacturing union has signalled his determination to claw back the ground lost by the trade union movement in the boardrooms of Britain and the corridors of power.

Derek Simpson, who begins work this week as the joint general secretary of Amicus, attacked the "exuberance" of ministers in "slapping down" unions.

He also set out his opposition to "sweetheart deals" between unions and companies, arguing that "competing to provide a worse agreement for our members should give way to a competition of who could provide the best".

Mr Simpson, who has been portrayed as an old-style leftwinger, made clear that he was no Marxist class warrior looking for battle.

"People don't say 'let's have a strike, let's have a week off'," he said. "They go on strike when, despite all the best attempts of themselves and the professional advisers, the trade unions are unable to reach amicable agreement."

He said he was determined to map out a role as a constructive critic within the Labour party. "I think first of all I need to set the record straight. People are trying to suggest that I'm attacking Labour or attacking New Labour. This is not the case."

The Amicus general secretary, who presides over the union in partnership with Roger Lyons, was not exactly fulsome in his praise of the Labour leadership.

Asked who should succeed Tony Blair as prime minister he replied: "It's a bit like asking which horse do you think will win the Derby, but we are not really looking at Derby-class horses here when you look at the potential candidates."

Rather ominously for Mr Blair, one of Britain's most powerful trade union figures offered a little more praise for Gordon Brown, his rival. "I get the impression that there's a more realistic and reasoned approach to trade unions, without the rhetoric, from Gordon Brown than perhaps from Tony Blair. Blair speaks perhaps in a more fraternal way, but do you believe the person who's putting on the face as much as another person who's talking more clearly?"

Mr Simpson noted warily: "I get the impression that unions are all right if they don't want to open their mouth and say anything."

He grudgingly welcomed the 1999 Employment Relations Act, which introduced compulsory union recognition if a certain number of workers supported it.

But he complained: "It's like drawing teeth. It comes diluted and as late as we can, like the rest of the legislation I complain about."

Mr Simpson acknowledged, however, that compulsory union recognition had increased the power of unions by leaving them less reliant on trying to gain voluntary recognition from companies through sweetheart deals that betray the interests of members.

He repeated his promise to "rip up" existing deals if members vote against them, estimating there were "probably about 10 of those that are questioned quite frequently that one would expect would be scrutinised". Mr Simpson hinted that some involved Japanese car companies - a comment that will worry Nissan and Toyota, with whom Amicus has agreements.