Labour national executive member Dennis Skinner warned yesterday that the party must be ready to reverse its attack on the trade union link if membership drops disastrously.

He predicted that only a fraction of trade union members would join up as individual affiliated members, leading to a serious financial crisis.

The veteran Labour MP pleaded with party leaders to pledge a full-scale review of the party's sweeping constitutional changes at the end of the planned five-year transition.

He attempted to persuade Tuesday's meeting of the party executive to make a clear commitment to a formal review - but without success.

Lord Collins, architect of the constitutional review, tamely suggested in reply that the party would anyway keep the implementation of his proposals under "continuing review."

Mr Skinner launched a blistering attack on the leadership's plan to ask individuals to opt-in as affiliated members, instead of unions affiliating members en bloc.

"The last time there was an opt-in was in 1927 after the General Strike, when the Tory government introduced a law to rub the workers' noses in it," he is reported to have told his fellow executive members.

He warned the party that it was "going back to 1927."

And he accused the leadership of kowtowing to the Tory press.

Mr Skinner said yesterday that he feared a huge drop in income from unions under the new opt-in system.

Just 238,000 affiliated trade union members voted in the 2010 leadership election out of a total of 2.7 million. It is believed that only about 150,000 of these voters were not also individual members of the party.

When a Tory government enforced the opt-in principle in 1927, Labour Party affiliated membership slumped by 1.2m to just over 2m.

In 1946 the Attlee Labour government re-introduced the opt-out and affiliated membership soared from 2.6m to nearly 4.4m in just one year.

The Collins proposals were approved by 28 votes to two at Tuesday's executive meeting, with Mr Skinner and Christine Shawcroft the only votes against, and Unite member Martin Meyer abstaining.