The warning came from one of Labour's biggest affiliates, the Amalgamated Engineering and Electrical Union, ahead of a secret meeting today of other big unions, which want to retain voting power in party elections and oppose the principle of one member, one vote (Omov).
The Labour leader has indicated that he might be prepared to shift ground on the issue since it became clear he would be defeated by the union block vote at the annual policy-making conference in October.
Mr Smith will be under pressure to clarify his position when he speaks to the Fire Brigades Union's annual conference in Bridlington on Friday.
Bill Jordan, president of the AEEU, accused the other unions of a 'conspiracy' and said dilution of a simple commitment to Omov would 'stand in the way of victory at the next election'.
He said: 'It really is time the union barons started to look to the real interests of the Labour Party instead of trying to protect past privileges.'
The secret meeting today has been called by John Edmonds, general secretary of the GMB general union - the party's second biggest affiliate, which is seeking to co-ordinate opposition to Omov and draw up an alternative. The only major union not to be invited is the AEEU.
The GMB has floated the idea that trade unionists should have a vote in the election of party leaders and the selection of MPs if they register as 'supporters' of Labour.
In an article last week in Labour News, Mr Smith, a GMB- sponsored MP, indicated that he might be prepared to consider such an option.
While union opponents of Omov are virtually certain of victory at the party's annual conference, so far there has been no consensus about an alternative.
Unions command a 40 per cent block vote in Labour leadership elections and varied block vote in the selection of parliamentary candidates. They will wield 70 per cent of the vote at the party conference, reduced from 90 per cent.
John Monks, deputy general secretary of the Trades Union Congress, yesterday won the AEEU's backing to replace Norman Willis as TUC leader after congress in September. No other contender has emerged since Mr Willis announced his retirement last month.Reuse content