The Scottish government’s narrative of improving exam results and attainment is “not a fully accurate picture”, MSPs heard this afternoon.

Conservative education spokeswoman Liz Smith made that argument as she pointed to Higher passes falling for several years in a row, and to research published today which suggests that the “least able” students have suffered most from problems with new qualifications.

Education secretary John Swinney rejected the claims and said there was a “broad set of data” showing that the attainment gap between pupils from more and less affluent areas was closing.

He pointed to a reduction in the gap at Level 6 (Higher or equivalent) for eight years in a row. He also highlighted improving “positive destinations” figures, among other data.

Secondary subjects: Subject uptake reduces across ‘entire senior phase’

Related: ‘Narrowing’ of curriculum limiting pupil choice, politicians told

Positive destinations: What do these figures tell us?

Background: What is Curriculum for Excellence?

Poverty: An open letter to teachers from a parent in poverty

Mr Swinney also gave details of an upcoming independent review of the senior phase of secondary education, which will be led by the Organisation for Economic Cooperation and Development (OECD), with a leading role also taken by education director and former headteacher Tony McDaid.

Is Curriculum for Excellence working?

Tes Scotland reported earlier today on a freedom of information (FOI) request which revealed that, six months ago, Mr Swinney was told that the number of subjects studied by pupils by the time they left school had reduced across the entire senior phase, from S4-6.

Meanwhile, a new report, also published in the hours before this afternoon’s debate, claims that Curriculum for Excellence has had a “significant negative impact”.

Professor Jim Scott, of the University of Dundee, found that attainment in levels 3-5 among S4 pupils had dropped sharply since the introduction of CfE.

He also found that the number of Higher passes in S5 had dropped by 10 per cent in the past four years.

According to Professor Scott, the worst affected pupils were those who are “less than average”.

In the report, he says: “Least able and ‘lower-average’ learners have suffered to a significantly greater extent than the able, and particularly the most able, in S4.”

Scottish Labour education spokesman Iain Gray said: “It’s time the SNP face up to their failings regarding their implementation of Curriculum for Excellence before more children have their future damaged by a government too embarrassed to admit its failings.

“Only by taking a critical look at the issues hindering the success of CfE, and by listening to teachers and students, can we return Scottish education to the standard it once enjoyed.”

A Scottish government spokesman said: “Improving the education and life chances of all our children and young people - irrespective of their background - is one of the defining missions of the Scottish government.

“That’s why we are investing £750 million during this Parliament to ensure every child has an equal chance to succeed.

“The gap between the most and least deprived communities for young people entering work, training or further study is half what it was in 2009-10, while a record number of students from the most disadvantaged areas gained a place at university last year.”