Achieving the class size target by 2011 – the end of its parliamentary term – was one of the SNP's most important manifesto commitments at the 2007 Scottish election and was judged to have played an important role in Alex Salmond's party rising to power.
Russell said there was now an "air of realism" about the policy, conceding that only the "bulk" of the target would be achieved by the end of the next parliament.
The failure of the SNP Government to deliver the policy contributed to the demise of Fiona Hyslop, who was sacked as Education Secretary when it became clear that she was facing a no-confidence vote at Holyrood.
As her successor, Russell's more realistic approach is clearly an attempt to repair the Government's strained relations with local councils, many of whom were having difficulty financing the extra teachers required to implement the policy during the recession.
Currently only 13 per cent of P1 to P3 children in Scotland are taught in classes of 18 or fewer.
Shortly after his appointment as Education Secretary, Russell offered an olive branch to councils by watering down the policy. He reduced the target to just one in five children aged five, six and seven being taught in classes of 18 by August this year.
Yesterday, however, he back-tracked even further when he admitted that meeting the target by 2015 could even prove impossible.
"Getting to 20 per cent by August 2010 is a realistic aim, leaving us to do the bulk in the next parliament," Russell said.
Russell's opponents described the class-size pledge as a "con". Iain Gray, the Scottish Labour leader, said: "It is the nail in the coffin of their last remaining flagship promise.
"This is a personal humiliation to the First Minister, who made this commitment in the Scottish Parliament.
"It was fundamentally dishonest for the SNP to make a pledge they knew they couldn't keep. The tragedy is that those who are suffering under the lost years of the SNP's education policy are the pupils and teachers of Scotland."
Elizabeth Smith, the Conservative education spokeswoman, said: "At last the SNP has admitted that their class size pledge was a cynical con to buy votes. It was dead in the water from the outset."
Last year, Alex Salmond was criticised at First Minister's Questions for telling MSPs in 2007 that he could deliver his manifesto promise within the current parliament when civil service advice issued two months before said otherwise.
A document was produced showing that a senior civil servant had advised "ministers" it would take eight to ten years to deliver the pledge.