Rena Mitchell, president of the Association of Head Teachers in Scotland, which represents primary heads, said: "A lot of schools would find the 80 per cent target very hard and many of these will be schools that are trying very hard."
Mrs Mitchell, head of Forth primary in South Lanarkshire, also had doubts about aggregating the 5-14 levels to produce an 80 per cent attainment target for primaries.
"I know of schools which achieve almost 100 per cent attainment at level A but get very poor results at the upper stages. Is that to do with the teaching or unrealistic levels?
"This issue should be tackled first before national tests are used to set targets, something for which they were never intended."