A union campaign on equal pay in the public sector should be extended to make sure that it covers charity workers as well, according to prominent figures.

Critics in the voluntary sector criticised the campaign, which is being run by Unison, GMB and Unite.

The It's Time For Fair Pay campaign says councils should pay all staff a living wage of £7.20 an hour and offer a £1000 one-off payment in compensation for pay freezes.

However, Annie Gunner Logan, director of the Coalition of Care and Support Providers in Scotland, and Martin Sime, chief executive of the Scottish Council for Voluntary Organisations, said the campaign should be extended to third sector contractors.

In many cases, such organisations can pay little more than the minimum wage because council tendering exercises are driving down the hourly rate for services such as care at home, it is claimed.

Ms Gunner Logan has written to the trade unions, saying council workers are not the only ones facing low pay. "This situation is not restricted to public service workers employed directly by councils. It is also the case for those workers employed by voluntary organisations contracted by councils to provide public services. Indeed, the position of these workers is arguably more difficult," she said.

She added that there was significant union membership in the third sector and unions had a duty to campaign on their behalf as well.

However, a spokesman for Unison said the campaign only covered council staff.