Nicola Sturgeon warned teachers are being driven away

They acknowledge that 1,400 posts have been created thanks to Covid-19 funding from the Scottish Government and the Pupil Equity Fund.

But they stress these are temporary, with the majority due to cease by June, and warn that many have had to leave the profession.

READ MORE: Schools warned as fears grow over post-Easter 'exams'

John Swinney, Ms Sturgeon's deputy, is also accused of failing to follow up on concerns raised previously over the approach to recruitment in recent years.

It comes amid a growing debate over how to drive education recovery after most pupils were forced to learn remotely as a result of pandemic-related school closures.

Rejecting the idea of "catch-up classes", teaching unions say having sufficient capacity to provide one-to-one and "small group" support will be crucial. 

The letter to Ms Sturgeon states: "We are appalled to report that Local Authority budget cuts are resulting in even fewer posts being advertised, at a time when our pupils need our support most.

"In one council this year, announcements were made that 69 nursery teaching posts and 40 area cover teaching posts are to be cut and the staff redeployed, immediately filling all available posts.

"In another council, only supply work was advertised and interviews for this have just been cancelled.

It is planned that all pupils will return to schools full-time after Easter - but most were forced to learn online after campuses were closed to contain new Covid-19 variants.

"Over 1200 teachers submitted their application for this post and hundreds have been left disappointed that their application will simply be kept 'on file'.

"When approached, councils have advised that they already have a surplus of permanent staff that is greater than minimum requirement.

"The practice of filling classrooms to capacity and reducing the number of teachers to a skeleton staff is questionable, clearly in contempt of the GIRFEC values and The National Improvement Framework ideal to close the attainment gap in Scottish education.

"On page 52 of your 2007 manifesto, the SNP declared, 'It's time,' and pledged to cap P1-P3 classes at 18. This has never been fully achieved."

READ MORE: Teachers face rush to submit grades

Describing staff as being employed under "zero-hour temporary contracts whilst in pursuit of a rewarding professional career", the letter adds: "There are thousands of teachers who cannot secure mortgages or car loans, plan maternity leave or make long term commitments due the uncertainty of our employment.

"For those of us unlucky enough to be on temporary part time contracts, we have had to seek additional employment elsewhere to pay our bills.

"Many of our colleagues have even had to leave the profession due to the instability of employment.

"Understandably, many of our colleagues have reported a decline in their mental health and increased stress and anxiety levels due to the casualisation of the workforce.

"Despite this, we do appreciate the value we bring when we are able to work and continue with a smile on our face and provide the fantastic education and support our children need and deserve at this difficult time."

John Swinney has been accused of failing to follow up on concerns over recruitment.

The teachers have received support from Scottish Liberal Democrat leader Willie Rennie.

He said: "When I raised this with the First Minister in parliament, I was told it was all fine. But this letter proves that the casualisation of the teaching workforce is a very real problem. 

"Teachers moved mountains during the pandemic. They worked through impossible circumstances, to give children and young people the best education they could. It's a disgrace that thousands now can't be guaranteed a job for them next year. 

READ MORE: Private schools leader says majority of secondary pupils returned

"Just as we needed everyone available to boost our NHS last year, we need every teaching talent to support the education bounce back.  

"Scottish Liberal Democrats have a bounce back plan for education, and that starts with a teacher job guarantee for August. No teacher should be unemployed or feel underemployed."

During a recent meeting of Holyrood's Education and Skills Committee, Mr Swinney told MSPs that funding for the additional teachers had been provided for this school year and next.

He added that there was “no reason why teachers who have been part of the additional cohort that has been taken on – or part of any additional cohort that has been taken on in the context of Covid – should face any uncertainty about their position, because the finance is available to enable local authorities to offer continuity of employment”.

Ms Sturgeon said previously she would encourage councils to provide permanent jobs.