SCOTLAND's biggest trade union will not be backing either a Yes or No vote in the independence referendum, it can be revealed.

In a motion to the union's UK delegate conference, Unison instead laid down a challenge to the Yes Scotland and Better Together campaigns on public services and workplace issues.

Backing neither side is similar to the position recently adopted by the country's second biggest union, Unite.

Unison is a predominantly public sector union and has around 160,000 members north of the Border.

Over the last few months, the Labour-affiliated trade union has embarked on a consultation with Scottish branch members on the referendum and has conducted polling. It is understood that the organisation's Scottish council agreed to back neither side in the constitutional poll at its February meeting.

This position has been reflected in Unison Scotland's motion to its UK conference in Brighton.

This states: "In the debate which surrounds the 2014 referendum, the Scottish people and Scottish public service workers will be asking questions on which option will be best for working people and their families in Scotland.

"The answers to these questions are likely to be less clearly defined and subject to significant differences of view across the spectrum of opinion in the country and the Unison membership.

"As such, Unison Scotland will continue to press the various parties and organisations in the debate on the issues of importance to our members at work, to the services they provide and the communities they live in, encouraging the fullest engagement of our members in the debates, while respecting the diversity of views of the members which they will register in the referendum."

A source said the motion was a "courtesy" as Unison had some of the most devolved structures of any trade union and had already decided its stance.

However, another motion listed in the provisional conference agenda, in the name of Lewisham Healthcare, takes a different stance.

It blasted the "significant damage" posed by "secession", adding: "Conference therefore resolves to campaign for a United Kingdom-wide referendum on this matter."

However, a Unison insider said this motion "probably won't even reach the conference floor".

Others unions have been accused of a stitch-up in how they have arrived at their referendum decisions.

Usdaw, which represents shop workers, backed a No vote at its Scottish conference in Dundee after inviting Better Together chairman Alistair Darling to address members. Nobody from the Yes campaign was invited.

The No position of the Communication Workers Union was reached after a motion was drafted by its UK executive.

A spokesman for Yes Scotland said: "We take encouragement from the knowledge that more and more trade unionists remain open to independence, and it is our job - and our serious intention - between now and September 18 to persuade them of the opportunities of Yes to build a fairer and more socially just Scotland, to protect and enhance workers' rights and to create new jobs by growing our economy in a responsible and sustainable way.

A Better Together spokesperson said: "With backing from the GMB, Usdaw, NUM, Community and Aslef, it's clear that there is strong support for Scotland remaining in the UK among tradez unions."