The number of teachers leaving the profession has nearly doubled since Labour came to power, an education expert says.
The numbers leaving peaked in 2001.
They are being driven out partly by the pressure of government reforms, according to a study carried out by Professor Alan Smithers of Liverpool University.
The other factors causing people to leave teaching are indiscipline and workload, he said.
Professor Smithers carried out the study for the Department for Education and Skills and it will be published on Friday.
But in a preview given to a conference of the General Teaching Council, Professor Smithers outlined his findings.
He said the numbers leaving rose after 1998 to a peak of 46,500 in 2001.
The numbers leaving then fell last year, to 41,800 but this was still two-thirds more than the average of the mid-1990s, which was 25,000.
Professor Smithers told BBC News Online many teachers who left said it was because of initiative overload or workload.
"There was a big difference in the impact of workload between primary and secondary schools," he said.
Of those leaving primary schools, 52% blamed their workload, while 39% cited government initiatives.
At secondary level, 39% blamed workload and 35% government initiatives.
Professor Smithers said the government was addressing the issue of workload in primary schools by bringing in more classroom assistants, but that he thought a better way forward was to have more teachers.
Fewer people are now leaving teaching.
Professor Smithers believes that this is because some of the government's reforms have bedded in.
And he noted that the biggest losses from teaching were from shortage subjects such as science or foreign languages where the teachers' skills are in high demand in the wider economy.
"To retain teachers, the government should be looking at workload, pupil behaviour and targeted measures," he said.
The government says around two-thirds of teachers stayed in the job
for more than 10 years and 70% for more than five.
Despite increases in departures since 1998, numbers quitting the profession had fallen last year, the Department for Education confirmed.
"Our strategy of better pay and more support for teachers is paying off," a spokeswoman said.
"There are now about 25,000 more teachers in post than there were in
"We are committed to reducing workload and bureaucracy through the national workload agreement and now have 89,000 more support staff.
"Having turned the tide of recruitment we are now focusing on retention as well by ensuring that the talents of teachers are properly used and rewarded and that teachers are supported when they take difficult decisions on behaviour and discipline."