Ministers were facing a growing backlash last night over a national replacement heating scheme for the elderly after it emerged thousands of pensioners may be left without heating this winter because of a backlog.

Age Concern Scotland is to write a protest letter to Communities Minister Stewart Maxwell after the waiting list for people in line for a new heating system hit 10,000.

The scheme was set up in 2001 by the Scottish Executive to give free central heating to people aged over 60.

But it has emerged that some 10,850 people have currently applied but do not yet have an installation date. It is thought that thousands of pensioners may be left without heating as a result of the problems. It is known that some elderly people are being forced to borrow portable heaters to see them through the winter.

Pensioners groups say the Scottish Government is unable to cope with demand with Scottish Gas installing the systems at a rate of 250 a week. Last week alone, twice that number applied for systems. Last night, Mr Maxwell promised a review to improve the service.

Helena Scott, Head of Policy and Research at Age Concern Scotland, said: "This is unacceptable at this time of the year, the winter, as it will put older people a risk. There should not be this waiting list.

"We believe this matter should be urgently addressed because it really undermines the credibility of the central heating programme.

"It is a concern that there there are in excess of 10,000 people eligible for central heating and a substantial number of older people have been told not to expect anything till April or May next year. Doing 1000 installations a month is not enough.

"This is particularly concerning for these people who are eligible for the programme, and their present heating has been switched off due to being dangerous or at risk. There is possibly no alternative heating source made available to them."

Mr Maxwell said they were a victim of their own success. "Due to the popularity of the scheme, unfortunately there are waiting lists and these have been a feature of this scheme since its inception in September 2001.

"The Scottish Government recognises the high demand and is working hard with other organisations involved to deal with these waiting lists. Applications requiring the installation of new or replacement systems are being dealt with as quickly as possible.

"In response to this we are installing an extra 1500 systems on top of the 12,000 programmed for this year and we are currently carrying out an internal review to improve the operation of the scheme."

A spokesman for Communities Scotland, the Scottish Government agency which helps administer the scheme, said that some £50m would have been invested this year in installing 13,500 central heating systems in the homes of the vulnerable.