The campaign is designed to shake employers into action

A new advertising campaign, What Century is This?, is aiming to shame employers into closing the pay divide.

Pressure group Close the Gap, which launched the drive in Glasgow, argues the pay gap "belongs to the Dark Ages".

Meanwhile, trade unions have said they are running out of patience with local authorities on implementing a long-standing agreement on equal pay.

Some are considering going to the courts, and it has been warned that the deal could cost Scottish councils �500m.

Legal action

The single status agreement was signed in 1997 in an attempt to wipe out discrepancies in pay for thousands of council workers.

However, South Lanarkshire is the only local authority to have reached an agreement so far.

Now the Transport and General Workers Union, the GMB and Unison are all threatening legal action against councils.

If the councils believe that they can't honestly afford it themselves then they should be knocking on the door of central government Tommy Campbell T&G

They say local authorities face the real possibility of solicitors pursuing individual claims - and that the cost to councils could be up to �500m.

Tommy Campbell of the T&G said: "We want central government to intervene here.

"If the councils believe that they can't honestly afford it themselves then they should be knocking on the door of central government and we would support them knocking on that door."

Cosla, the body which represents Scottish councils, says negotiations have been highly complex and time-consuming.

However, it expects a big push towards the end of this year to try to reach agreement.

Inflatable billboards

Close the Gap launched its campaign, under which about 33,000 employers are being sent information on how to bridge the pay gap and avoid discrimination claims, on Tuesday.

There will be inflatable billboards in Glasgow, Edinburgh, Inverness, Aberdeen, Dundee, Perth and Stirling as part of the campaign.

Close the Gap says YouGov surveyed 206 people from Scotland on 24 February and found that 28% of those were unaware a gender pay gap existed.

Scotland has a gender pay gap that belongs in the Dark Ages Emma Ritch Close the Gap

And 29% of women interviewed said they do not believe they will ever earn the same as men.

Emma Ritch, of Close the Gap, said: "Our campaign highlights the fact that even though it is 2005, Scotland has a gender pay gap that belongs in the Dark Ages.

"What Century is This? works with trade unions, employees and employers to raise awareness of the fact that paying women less than men is illegal and that the pay gap should not be a fact of life."

Scotland's gender pay gap is "a national scandal" which needs to be addressed, she added.

'Fair deal'

Deputy Communities Minister Johann Lamont said: "The pay gap between men and women is decreasing but not fast enough. It is unacceptable that at the beginning of the 21st Century women should still be earning on average 15% less than men's earnings.

"Tackling equal pay must be a priority for Scotland's employers - women's work should be properly valued and women should expect a fair deal in the workplace."

The Scottish Trades Union Congress said the campaign was a positive step in raising awareness about the pay gap and encouraging workers to take action.