The warning - and renewed confirmation that Mr Smith will not give in on the issue - came after Tom Sawyer, deputy general secretary of Nupe, and Robin Cook, Labour's trade and industry spokesman, suggested unions might accept one member, one vote (Omov) for parliamentary selections if they could keep the old system for leadership elections.
Mr Smith has repeatedly insisted he will not back down over his plans to allow individual trade unionist participation in both contests if they topped up their political levy payment with cut-price party membership - the so-called 'levy plus' principle.
Mr Cook managed Mr Smith's leadership campaign last July and would not have spoken out in defiance of a specific instruction from Mr Smith, indicating that the high command has no objection to testing the strength of feeling in favour of a compromise.
Mr Cook supports the Omov principle but felt able yesterday to highlight how a third of a million trade unionists had cast 'real votes on real ballot papers' last year in those unions who consulted their memberships over the new leadership.
'I proposed then that it would be sensible to have a register of Labour supporters so that those trade union members who wanted to could sign up as registered supporters and could then exercise a vote in future leadership elections,' he told BBC television's On the Record.
A registered supporter system appeals to some union chiefs as they would remain the conduit of the membership's views, so retaining clout in the party.
But a compromise deal partly invoking it, which has hardened as a possible solution over the past week, was strongly rejected by one of Mr Smith's parliamentary supporters yesterday.
'You cannot back off on one and split the difference,' he said.
'We believe trade unions should exercise power within the party, not outside it, not as a separate category.
'No other political party has non- members selecting candidates or its leader. You would be substituting one trade union block vote for another.'
Unions will wield 70 per cent of the votes on the issue when it reaches the party's autumn conference and Mr Smith could easily be outvoted.
But Mr Smith is prepared to see figures such as John Edmonds, general secretary of the GMB, his sponsoring union, face the embarrassment of having to vote against him publicly.
Mr Edmonds showed himself unwilling to compromise on Friday as he took steps away from the registered supporter idea, demanding no less than allowing all political levy payers to vote in parliamentary and leadership elections.
These outnumbered the party's 200,000 mainly middle-class members by 20 to one, he said.
The level of disaffection between Mr Edmonds and Mr Smith is highlighted by the union's failure to invite the Labour leader to speak at its forthcoming conference.Reuse content