The number of primary school children making good progress in reading, writing and maths has fallen sharply over the last 12 months, after the government changed its tests.
In 2015 84 per cent of 11-year-olds reached Level 4 in the SATS tests.
But after a government shake-up designed to make the trests more challenging only 59 per cent achieved the expected standard.
Headteachers have made a string of criticisms about the changed tests.
The National Association of Head Teachers has dismissed the results are “not worth the paper it is written on,” pointing to delayed guidance for schools, mistakes in test papers, and answers being accidentally published online.
While all education authorities saw their overall figures drop, Herts also fell compared to other authorities.
In 2015 the county was 27th out of 152 councils, but this year was 31st.
Councillor David Williams, portfolio holder for enterprise, education and skills at Herts County Council, said the results were still positive as it kept local results above the national average.
He said: “We are immensely proud of our children in Hertfordshire. 59 per cent of pupils in the county achieved the new more challenging standards for Key Stage 2 in reading, writing and mathematics compared to 53 per cent of pupils nationally.
“It is a credit to everyone involved in education in Hertfordshire that our primary schools continue to outperform the national average.”