Mr Morgan, the MP for Cardiff West, quickly welcomed the decision.

Unison is giving its members a free vote

"This is good news for democracy," he said. "This is the kind of poll which you cannot argue against and I shall be glad to win or lose on the strength of it."

Unison is the biggest union in Wales to back a ballot rather than merely "consult' its membership and then cast a block vote in the forthcoming contest between Mr Morgan and new Welsh Secretary Alun Michael.

Mr Michael has the backing of Tony Blair, whereas Mr Morgan - the front-runner - is seen as the grassroots choice.

Mr Morgan said: "Unison members will be able to compare and contrast Alun Michael's leadership with my own."

Electoral college

Under Labour Party rules, unions and other affiliated organisations have a third of the votes in the electoral college to be held in February to choose Labour candidate for First Secretary of the Welsh Assembly - Wales's first "prime minister".

Constituency parties and MPs and selected assembly candidates make up the other two voting sections.

Two other unions, the Manufacturing, Science and Finance (MSF) and the National Union of Mineworkers, have also indicated they will also hold their own ballot.

Announcing the ballot move, Derek Gregory, Welsh regional secretary of Unison, said the union was responding to calls from members.

"It's a decision of principle, based on the feelings of our members. We have to take into account what our members and branches are saying to us," he said.

"If representative democracy is good for the House of Commons, it's also good for the trade unions. We want our members to participate in this important decision".

'No threat to Alun'

Don Touhig: Supporting Alun Michael

Don Touhig MP, secretary of the Wales Labour Party "task force" that drew up the rules for the contest, insisted Alun Michael would have nothing to fear from Unison's decision to hold an individual ballot.

"He was a councillor for many years; he worked in the voluntary sector and knows a great many people in Unison," said Mr Touhig in a BBC Radio Wales interview.

"Each candidate will have a chance to put their policies and proposals before the rank and file of the party, the trades unions and others.