Mr Salmond said problems with the scheme were being rectified

She told MSPs average waiting times for installation had doubled under the SNP, from 114 days to 229 days.

During question time at Holyrood, First Minister Alex Salmond said the government was addressing problems left by the previous administration.

He said the scheme his ministers had inherited was not acceptable.

Ms Alexander said that when the SNP took power after the election last May, barely 1,000 pensioners were waiting more than three months for their free central heating system.

Nobody is pretending that scheme is working effectively Alex Salmond First Minister

She said figure had risen to more than 6,500, adding: "The reality for those pensioners is that they have to choose either to freeze, or to pay up for themselves."

Some 15,000 systems, Ms Alexander said, had been promised by April, but the latest figures showed the government had barely managed 8,000 by the end December.

The Labour leader demanded: "Is the first minister now willing to apologise to those pensioners who will be freezing in February because of his failure?"

Mr Salmond said action being taken to repair deficiencies in the scheme, inherited from the previous Labour/Liberal Democrat administration, was bringing substantial improvements.

"Nobody is pretending that scheme is working effectively," he said.

The first minister went on: "We don't think the scheme we inherited is acceptable - which is precisely why we have introduced 1,500 extra places over the course of this winter, which will result in a record 15,000 installations.