Labour leader Ed Miliband faced stark warnings yesterday that only radical policies can save his party following the dangerous plunge toward a weaker relationship with the unions.

Leading labour movement figures were sharply divided over plans for sweeping organisational changes, which include asking trade unionists to "opt in" individually as affiliated party members.

But trade union leaders and left MPs united in warning that without radical policies the party is likely to face financial crisis and the threat of defeat at the next election.

Unite general secretary Len McCluskey declared that the proposed constitutional changes had the potential to involve hundreds of thousands of ordinary trade unionists in Labour Party activities.

Mr McCluskey said his union had "engaged enthusiastically" in discussions about the changes proposed in a report from former Labour Party general secretary Lord Collins.

But he added that the "kernel" of the matter was whether the party could come up with radical alternative policies to inspire trade unionists.

"How do you persuade trade unionists to be more active in the Labour Party? Well, they have to be enthused," said Mr McCluskey.

The proposed changes were bitterly condemned by Campaign for Labour Party Democracy secretary Pete Willsman.

Mr Willsman pointed to similarities between the proposed opt-in of trade unionists and the "vicious, vindictive" law introduced by a Tory government in 1927 to punish the labour movement after the 1926 General Strike.

"They have not learnt the lessons of history," he warned, recalling that the 1945 Attlee government had restored the rule that trade union members must opt out if they did not wish to pay the Labour affiliation fee.

Senior Labour backbencher Ian Davidson MP angrily protested that weakening the trade union link was "unfinished business from the Blairite years."

Labour MP Ian Lavery warned that the party "now faced its biggest challenge in many years in terms of alternative funding arrangements."

He said the Collins review would be "received with mixed political emotions from the wider labour and trade union movement."

Mr Lavery was involved in talks on the constitutional changes in his role as chair of the trade union group of Labour MPs.

But he emphasised that trade unionists and others would only support the party financially if it came up with "bold" policies and manifesto commitments.

The Collins proposals will be debated by MPs at a meeting of the Parliamentary Labour Party tonight and by the party national executive tomorrow. They will be put to a special party conference on March 1.

Some Blairites are upset that MPs would no longer have one third of the vote in the electoral college for leadership elections.

However, former political secretary to Tony Blair John McTernan tweeted: "Ed Miliband is more Blairite than Blair on party reform."

MPs would have just one vote each, but the chances of a left-wing candidate would be severely undermined by a new rule that candidates must have the support of at least 20 per cent of MPs, instead of 12.5 per cent at present.

Labour deputy leader Harriet Harman confirmed yesterday that no amendments will be allowed at the March 1 party conference.

"It will not be a rubber stamp, because people can say no," she argued.

Ms Harman claimed that the new affiliation fee of around £3 per year for trade union members would "open up the Labour Party to people at workplaces up and down the country."

She suggested that around 270,000 trade unionists could be persuaded to join up, adding to around 180,000 full party members who pay up to £45.50 per year.

At present, the affiliated trade union membership is around 2.7 million.

General union GMB general secretary Paul Kenny said the plans would have "financial implications for the Labour Party."

How many trade unionists opt in would depend on the popularity of Labour's policies.