The use of new primary school tests to help judge national progress in literacy and numeracy will not be reliable until at least 2021 - four years after they were introduced, MSPs have heard.

John Swinney, the Education Secretary, made the admission during an evidence session of the Scottish Parliament’s education committee, which is holding an inquiry into the assessments.

Tavish Scott, education spokesman for the Scottish Liberal Democrats, said the committee had heard evidence in previous sessions that the statistics so far were “experimental”, with only one third of teachers confident of the robustness of the data.

“By definition two thirds were not confident. How can you be so confident you are building up that national picture?” Mr Scott asked.

Mr Swinney responded: “As with all data there is a period you have to go through to get to a point where statisticians will give you that appropriate standard and we are on course.

“My expectation is that probably in the next academic year, 2019/20, we would see that.”

Mr Scott responded: “So 2019/20 will be the first academic year that we will have a set of figures for the performance of Scotland’s national education system which we would then be able to compare in 2021.

“In terms of all the interventions that have been made since national testing was introduced, it will be three, four or five years before we can assess what has changed.”

Use of the assessments to measure progress in closing the school attainment gap between rich and poor also came under fire at the committee because the assessments have not been adopted by private schools.

Iain Gray, education spokesman for the Scottish Labour Party, said a previous survey, which the Scottish Government scrapped, was a better judge of whether schools were closing the attainment gap because it also involved schools in the independent sector.

Mr Gray said: “Given the government’s main objective is to close the attainment gap could you explain how removing a cohort largely from the more privileged end can help to do that?”

Mr Swinney said the assessments would help judge the closure of the attainment gap in the “public” part of the education system.

So far this year 144,941 standardised assessments have been sat compared to 119,616 last year.