Britain's unions are threatening to deliver a fresh snub to Tony Blair by ditching their support for UK entry to the single European currency.

Left wing delegates at the Trades Union Congress in Blackpool are hoping to overturn more than a decade of union support for the euro in what is expected to be a fierce debate on Wednesday afternoon.

Such a move would deal a severe blow to Mr Blair's hopes of holding a euro referendum in the current Parliament.

In his speech to the TUC conference on Tuesday, Mr Blair once again stressed that Britain should join the euro if the economic tests were met.

He said it would be a "vast error" for Britain to turn its back on Europe.

"For Britain to be marginalised in Europe when soon the EU will have 25 members stretching from Portugal to Poland and the largest commercial market in the world, would not just be economically unwise," he said.

Ally

"It would betray a total misunderstanding of the concept of national interest in the 21st century."

TUC General Secretary John Monks has been a staunch ally of Mr Blair in the euro debate, arguing it will save jobs.

But left-wing unions, including Unison, fear the effect joining the euro would have on the National Health Service and other public services.

European rules on public spending enshrined in the EU Stability and Growth Pact would, they fear, rule big increases in public spending.

Ballot

Of the big four unions, only the GMB is expected to back motion calling for British entry if the five economic tests are met.

TGWU leader Bill Morris wants a referendum delayed until the next Parliament.

And the new leader of engineering union Amicus, Derek Simpson, has said he might hold a ballot of members on the issue.

An ICM poll for the anti-euro No Campaign recently suggested the majority of trade union members would vote against the euro, by a ratio of 49% to 32%.

It also found only one in 20 union members thought the euro was an important issue.