Education Secretary John Swinney faces increasing pressure to scrap controversial tests for primary one pupils as he faces a national boycott campaign.

Campaign group Upstart Scotland is distributing 30,000 postcards urging parents with children starting P1 this week to sign up to withdrawing from the literacy and numeracy assessments, which teachers say have left some pupils in tears.

Teaching union the Educational Institute of Scotland is backing the move, having earlier called for the tests to be scrapped, and other supporters include the parents’ group Connect and charities Play Scotland and Children in Scotland.

Parents are being urged to sign the postcards, fill in their child’s name and send them to the head teacher.

The postcards state: “I do not want my child to sit the primary 1 tests of literacy and numeracy.

“I firmly believe that national standardised assessment of this kind is not developmentally appropriate for young children and would, therefore, prefer assessment to be based on teacher observation and professional judgment, in accordance with the Early Level of the Curriculum for Excellence.”

Scottish national standardised assessments (SNSA) were introduced across four age groups to help measure the attainment gap in schools.

Mr Swinney has said guidance for the assessments makes it clear they should not cause distress but he faces cross-party opposition to the P1 tests.

Scottish Labour education spokesman Iain Gray said: “Schools across Scotland are returning to an SNP omnishambles – a testing policy descending into chaos as it faces a major campaign by parents and a potential teacher boycott, hundreds of unfilled posts across the country and teachers on the verge of industrial action.

“John Swinney must suspend these tests, he cannot keep defending a discredited policy that is opposed by parents pupils, teachers and the unions.

“These tests are driving children to tears and do nothing to help teachers close the attainment gap. It’s time for the SNP government to finally see sense.”

Scottish Conservative leader Ruth Davidson also called for the P1 assessments to be scrapped “without delay”, saying she agreed with EIS General Secretary Larry Flanagan who wants the tests to be dropped following concern from teachers.

Her Liberal Democrat counterpart, Willie Rennie, wants a parliamentary vote on abolishing the tests.

He said: “How many teachers and parents have to boycott these tests before the Education Secretary finally listens?”

A Scottish Government spokesman said changes and improvements will be announced soon following a review of the first year of assessments.

He said: “The Scottish National Standardised Assessments ensure for the first time that all schools will undertake the same assessments, providing consistency and an important means for teachers to identify children’s next steps in learning.

“That is especially valuable in early years if we are to continue to close the attainment gap.

“Our approach was developed after extensive engagement with teachers, parents, children and academics. Teachers have the flexibility to manage the assessments to ensure that they are a positive experience for all children.”