A BBC manager has publicly turned down a promotion after finding out she had been offered £12,000 less than a man doing the same job, threatening legal action against the broadcaster and suggesting the corporation is still struggling with equal pay.

Karen Martin emailed hundreds of BBC staff to announce she would no longer be taking up her role as one of the two deputy editors in the BBC’s radio newsroom, which produces material for broadcast to hundreds of millions of people on both UK radio stations and the World Service.

The experienced radio producer’s new job, in which she would oversee global radio news output, had been announced in February. But Martin said she could not accept the role after discovering the other newly appointed deputy editor, Roger Sawyer, had been offered a substantially higher salary.

“Despite being awarded the same job, on the same day, after the same board, during the same recruitment process, BBC News asked me to accept a considerably lower salary than my male counterpart. A lot less,” she wrote in the email sent to all BBC radio newsroom staff.

“I’ve been assured our roles and responsibilities are the same. I’ve also been told my appointment was ‘very well deserved’. It’s just that I’m worth £12,000 less. Over the past four months I have asked BBC News to think again. And they’ve inched their offer up by addressing historical ‘under payments’. Now the gap is nearer to £7,000. But for me it has never been about the actual salary. It has been about equal pay.”

Martin said her mind had been made up after talking to her daughters: “As a single mother I decided to discuss the dilemma with my two daughters who are 13 and seven. There were no long meetings, no need for reviews and no wrestling with consciences. There were no attempts to explain away the injustice.

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“They said: ‘You always tell us to stand up for what is right. If it means less pocket money or not going on holiday, we don’t mind. What matters to us is that when we grow up we want to be paid the same as a man for the same job.’ And just like that, my decision was made.”

The BBC is already facing an investigation by the Equality and Human Rights Commission into historic claims that men were consistently paid more than women for doing the same job. Martin said she would consider helping that inquiry and reserved the right to take legal action against the BBC.

She said she had been in touch with the BBC director general, Tony Hall, about the issue. Martin also said she had been warned that her career would be damaged by her decision to turn down the job.

The corporation has been forced to adjust many salaries following the resignation of Carrie Gracie as China editor after she complained about a “secretive and illegal” pay culture, in addition to the public disclosures about the high salaries paid to male presenters. Despite some progress, many prominent BBC women remain unhappy with how the corporation has handled the issue, with several cases outstanding.

The head of BBC news output, Gavin Allen, later emailed staff in a bid to reassure them about Martin’s situation, saying the BBC pays staff according to broad salary bands agreed with unions rather than setting specific salaries for individual jobs.

“Roger has worked at or above this level for several years whereas Karen was offered this role as a promotion, with a significant pay increase. We think most people would understand that these factors would result in some difference between their individual pay,” he said.

“I accept that we have not always got things right in the past on pay but I believe this is not one of those cases.”

It is understood that the relevant pay band in this instance is £60,000 to £100,000.