The EOC said the enquiry would concentrate on the Royal Mail business unit, which includes the mail sorting offices and postmen and postwomen.
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The EOC has serious concerns about the numbers of allegations of harassment in the Royal Mail's sorting offices and against postwomen
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Equal Opportunities Commission
It follows what the EOC called "extensive correspondence" from employees.
"There are grounds to believe that RMG has failed to take sufficient steps at national, regional or local level to prevent significant sexual harassment of its female employees over a sustained period of time," said the EOC.
Changing culture
In the past three years, there have been 140 internal claims of sexual harassment made by female postal workers.
Of these, 130 led to internal investigations and 65% of the complaints were upheld.
The EOC said it assisted in three of these cases which resulted in the either a substantial settlement or imprisonment.
But the commission said it was still not satisfied that enough was being done.
"The EOC has serious concerns about the numbers of allegations of harassment in the Royal Mail's sorting offices and against postwomen and the way in which some of these have been handled," said Jenny Watson, deputy chair of the EOC.
No progress?
Royal Mail chairman Allan Leighton said he supported the investigation and welcomed it as "an opportunity to stamp out sexual harassment within the company".
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Our previous attempts to tackle the problem have not delivered results
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Jenny Watson, EOC
Mr Leighton even went so far as to say it was a good business move.
"This behaviour is very high on the new Royal Mail Board's agenda firstly because... it is totally unacceptable and secondly because it makes good business sense," he said.
However, the EOC said it has received 22 complaints in the last three years that included an allegation that management failed to investigate the issue fairly, or even at all.
"We believe that, at board level, they are committed to changing the workplace culture," said Mr Watson.
"However, our previous attempts to tackle the problem have not delivered results."
Earlier this month, the Royal Mail installed a free 24-hour helpline in an attempt to stop workers being bullied.
The Communications Workers Union said it had received more than 9,000 calls to its counselling service in the past two years and Royal Mail admitted it had "problem with bullying".