Michael Gove has lead a pre-conference season assault on Labour leader Ed Miliband, accusing him of allowing trade union influence back into the front line of his party.
In a series of attacks, Gove told an audience at the Conservative Party's headquarters in London that far from reducing union influence over him, Miliband's proposed reforms to the relationship with Labour will increase the problem.
The Education Secretary said Labour is "sinking back into their pre-Blair position of living in the unions' shadow".
Miliband has pledged fundamental reform of the unions' relationship with the party in the wake of the controversy over Unite's involvement in candidate selection in Falkirk.
The party's biggest donor was accused of signing up members in the constituency so it could influence the choice of a candidate.
The Labour leader is set for a rough ride over the reforms when he addresses the TUC annual conference next month in what is expected to be his first major speech after a bruising parliamentary recess.
Critics, including several senior party figures, have complained that the party has failed to get its case across to the public or to hold the Conservatives to account over the long break.
Gove urged Mr Miliband to go further on his reforms, working with the Government to change the law on third party campaigning in politics and to allow union members to direct the political levy to a party of their choosing.
Gove told the audience: "Tony Blair once argued that the Labour Party should not be the political arm of the trade union movement but the political movement of the British nation as a whole.
"That's what One Nation politics means," he added in a pointed reference to the central theme of Miliband's speech to the Labour Party conference last year.
"But, sadly, Labour are now sinking back into their pre-Blair position of living in the unions' shadow."
Miliband was elected on the back of trade union votes "because they believed he would be the most pliant personality available" and had been proved right, he said.
"With him in place, radical left-wing union leaders now believe the Labour Party can be theirs again, and they are taking it back seat by seat, policy by policy, before his impotent gaze."
He had "failed to act - and has no plans to act" to change selection procedures, investigate other claims of Unite unduly influencing the choice of candidates or other issues raised by the Falkirk case.
Comparing him with another of his predecessors, Gove said: "While (Neil) Kinnock moved bravely and remorselessly to eradicate Militant's influence and Militant-sponsored MPs from Labour, Miliband has done nothing to stop the takeover of his own party.
"The sad truth is that - charming, intelligent, eloquent, thoughtful, generous and chivalrous as Ed Miliband may be - in this critical test of leadership he has been uncertain, irresolute, weak.
"To the question 'Who governs Labour?' his answer would appear to be, increasingly, the unions.
"And if Ed Miliband is too weak stand up to the union bosses who pick his candidates, buy his policies and anointed him leader, then he simply will be too weak to stand up for hard-working people.
"Our country cannot afford - as we had in the '70s - the same old Labour Party with a weak leader buffeted by union pressure to adopt policies only they want and asking hard-working people to pay the bills."
A Labour Party spokeswoman said: "Ed Miliband's party reforms are the biggest for a generation, since John Smith introduced One-Member-One-Vote. They will modernise and strengthen Labour's historic relationship with trade unions.
"Michael Gove should be explaining why the Tories are standing up for the wrong people, why after holding secret Downing Street dinners for big Tory donors David Cameron made policy concessions to the wealthy few, and why the Tories have failed to answer straight questions about the role of Lynton Crosby, who combines his job as a lobbyist for the tobacco industry with being the Prime Minister's chief political strategist.
"Michael Gove, like David Cameron, has got too close to a wealthy few and too out of touch with everyone else."