Vacant teaching posts have risen by almost a quarter over the last two years, new figures suggest.
There are nine per cent more teaching vacancies this September than there were last year, according to figures from Eteach.
The education recruitment service said data from a quarter of the UK's schools shows there are 13,969 teacher vacancies at the start of the school year - an increase of almost 24 per cent from 11,275 in September 2015.
It analysed teacher vacancy figures from 7,000 schools and found that with an average UK class size of 22 pupils, at least 300,000 pupils would be without a permanent classroom teacher this September.
The service also surveyed more than 1,800 teachers and found that two thirds are thinking of leaving their current job in the next three years.
Excessive workload and low morale were given as some of the reasons for planned resignations, with 80 per cent of teachers taking work home with them on weekends and during holidays.
However, 56 per cent of those polled said they would still recommend the profession to a friend.
Paul Howells, founder and chief executive of Eteach, said: "In my 20 years of working with schools on teacher recruitment I have never seen so many unfilled vacancies.
"Teaching is failing to attract enough graduates, and schools are struggling to hold on to their current teachers, creating a perfect storm of pressures.
"As schools start the new academic year, thousands will be scrambling for last-minute supply staff and paying agencies a premium for emergency cover during a period of tightened budgets."
A Department for Education spokesman said: "These figures do not reflect the fact that the teacher vacancy rate in 2016 remained low - at 0.3 per cent. The number of teachers entering our classrooms is outnumbering those who retire or leave, and there are now more teachers in our schools than ever before - 15,500 more since 2010.
"And as announced at the 2015 Spending Review, we are investing £1.3 billion up to 2020 to attract new teachers into the profession."