The Department for Education’s concern about teacher shortages has been laid bare in a private email seen by Tes.

Unions have now called on the DfE to “come clean” and publicly admit the severity of the situation following the email in which it states:

• "Challenges in teacher supply have worsened".

• The overall number of teachers fell by 1.2 per cent between 2016 and 2017, while pupil numbers, especially in secondary schools, are projected to continue rising.

• The rate of teachers leaving teaching is now equal to the rate of teachers joining.

The DfE email sent just before Christmas comes in marked contrast to the public stance the department has been taking on the teacher recruitment crisis.

While acknowledging that schools are facing a recruitment "challenge", the education secretary Damian Hinds has chosen to emphasise the positive. In May he said: "Teacher numbers are at an all time high and more people are returning to teaching this year."

And in April a DfE statement said: “There are now a record number of teachers in our schools – 15,500 more than in 2010 – and the number of new teachers entering our classrooms outnumbers those who retire or leave.”

But, as last month's email acknowledges, by then numbers had already begun to fall and new joiners are not making up for the losses.

The email reads: "While the overall number of full-time equivalent teachers working in state-funded schools in England has remained broadly stable in recent years (falling by 1.2 per cent between 2016 and 2017), the rate of teachers leaving teaching is now equal to the rate of teachers joining.

"This is further problematised by the fact that (especially secondary) pupil numbers are projected to continue rising for the next few years."

Analysis by Tes last year found that 47,000 more secondary teachers were needed by 2024 to cope with an "explosion" in the number of pupils. At the time the DfE was asked to respond but did not publicly acknowledge the pressure that increased rolls would put on teacher supply.

Geoff Barton, general secretary of the Association of School and College Leaders, accused the DfE of “attempting to put a positive spin on its lamentable record over teacher recruitment."

He said: “It is true that the total number of teachers has risen since 2010 but this has to be understood in the context that the number of pupils in our schools has increased by more than 600,000 in that time.”

The DfE, which is about to launch a new teacher recruitment and retention strategy, sent out the email to private contractors last month inviting them to bid for a research contract exploring the extent to which teaching assistants have the appetite to become teachers.

Yesterday it emerged that DfE spending on TV teacher recruitment adverts cost £4,291 for every person who registered an interest online in teaching a shortage subject.

Chris Keates, general secretary of the NASUWT teaching union said the email showed “a crisis in teacher supply” despite the government trying to claim the contrary, and called on the government to “face up to the crisis it has created”.

She said: “It is the government’s own policies which have resulted in excessive and increasing teacher workloads, dwindling pay, starting salaries which are increasingly uncompetitive with other graduate professions and the relentless pressure of the high-stakes accountability regime.

"These factors are driving existing teachers out of the profession, sapped of energy and enthusiasm for the job, and deterring new entrants.”

Andrew Morris, assistant general secretary of the NEU teaching union said there was “a crisis in secondary teacher recruitment”.

He said: “For the sixth year in a row, the DfE has failed to recruit enough secondary teachers overall, based on its own targets, nor does it take into account the rise in pupil numbers.

"It is time for the DfE to come clean, admit the extent of the crisis in teacher recruitment and retention, and work fast to fix it. If we are to recruit and retain the teachers we need, government must look and act very quickly on the issues of pay, workload and ensuring all schools are properly funded.”

The DfE said its comments had been based on the latest available data and were correct at the time. A spokesman said: “The fact that more than 34,500 new trainee teachers started teacher training courses this year – over 2,600 more than in 2017 – despite a competitive labour market shows that teaching continues to be an attractive career.

“This summer we announced a 3.5 per cent increase to the main pay range for classroom teachers backed by a £508 million government grant. We also announced uplifts of 2 per cent to the upper pay range for higher-paid teachers and 1.5 per cent to the leadership pay range. This was the biggest pay award in almost 10 years.

“The education secretary has made it his top priority to cut unnecessary workload, but there is more to do which is why the department will shortly set out its recruitment and retention strategy to ensure teaching remains an attractive and fulfilling career.”