English schools are facing a "brain drain" of teachers because they are lured to private schools overseas, the head of Ofsted has warned.
The watchdog's chief inspector Sir Michael Wilshaw claims teachers should be given a "golden handcuffs" deal to stop them going abroad once they qualify.
His comments, in a monthly commentary on the Ofsted website, come as a study claims that a private education gives pupils a two-year advantage over their state school counterparts by the time they are 16.
The study, commissioned by the Independent Schools Council but carried out by researchers at the University of Durham, showed that private schooling pushed GCSE students an average of two thirds of a grade higher in each subject, according to reports.
Sir Michael has suggested that teacher shortages are being worsened by staff going to work abroad, particularly at campuses of elite British schools.
Last year more people left to teach abroad (18,000) in English language international schools than trained (17,000) on post-graduate routes, he said.
He said: "Anyone regularly perusing the job vacancy pages of the education press cannot help but notice just how many of our elite public schools are busy opening up international branches across the globe, especially in the Gulf States and the Far East.
"Two years ago, there were 29 of these overseas franchises.
"At the end of 2015, there were 44 and the number will rise again in the coming months with several new campuses scheduled to open soon."
But it is not unreasonable to ask teachers trained in the UK to commit to the country for the first few years of their career, he claims.
"I would, therefore, once again urge policymakers to consider the idea of some form of 'golden handcuffs' to keep teachers working in the state system that trained them for a period of time," he said.