Ms Forgan has told friends that she made the firm decision to quit when she was outflanked by Tony Hall, managing director of news and current affairs, on the issue of moving radio news headquarters to a new "bi-media" operation at Television Centre, four miles west.
She fought the plan bitterly but John Birt, the director-general, did not give her the support she hoped for. Ms Forgan decided to resign when governors ruled in Mr Hall's favour last August, but delayed announcing her departure until last week so as to minimise controversy. The move will happen next year.
To those not engaged in the parochial in-fighting of BBC politics, the issue might seem trivial. Mr Hall's department already has management authority over news and current affairs output on television and radio, including flagship programmes such as Today and The World Tonight. But to Ms Forgan, basing them at Broadcasting House, the Art Deco building in Portland Place where radio has been sited since 1932, symbolises that their first loyalty is to the radio service. The link will be weakened by the move.
"She felt it pulled the guts out of radio and diminished its importance,'' a friend said. "It sent a message that radio to the BBC was now simply an adjunct to television."
BBC top brass are frightened that her decision to go in April, only three years into her five-year contract, could harm fragile staff morale. Colin Browne, director of corporate affairs and a member of the board of management, spent hours last week on the telephone to journalists, in an unusual top- level attempt at spin-doctoring and damage limitation. Meanwhile Ms Forgan stayed strictly under wraps, even turning down an interview on her own Today programme.
There are suggestions that she will not be replaced, because Mr Birt and the governors are considering a plan to merge the radio and television directorates under a single managing director of programmes. A decision will be taken after the new chairman, Sir Christopher Bland, takes up his post in April.
While the defeat by Mr Hall triggered her departure, it was not the only factor in her creeping disillusion with the House That John Birt Built. Coming from small, informal organisations - Channel 4 and before that the Guardian - she felt weighed down by the BBC's wearying bureaucracy, the endless round of performance reviews, strategy meetings, sessions with accountants and management consultants.
Ms Forgan, the only woman among the BBC's six managing directors, is often described as a "free spirit" - a term meant as either a criticism or a compliment, depending on which of her colleagues is expressing it. When she arrived at Broadcasting House, she circulated a memo urging her staff to treat her office "like Waterloo Station"; but as she was sucked into the BBC's way of doing things it became almost as hard to make an appointment with her as the famously aloof Mr Birt.
She is not the first to have been ground down by the system: other Birt appointees who departed early include Ian Hargreaves and Polly Toynbee from news and current affairs, David Liddiment from entertainment and Nick Elliott from drama. Recently news and current affairs has also lost two longer-serving executives: Tim Gardham to the nascent Channel 5 and Chris Cramer to the American Cable News Network.
"The BBC is a very sad place," a Broadcasting House stalwart said after Ms Forgan's announcement. "We're losing someone who's been a champion of radio." Yet that was not the opinion when she arrived three years ago to hostility and sneers.
At the end of 1992, just before John Birt took over as director- general, there were rumours that he intended to make her his deputy. As director of programmes at Channel 4 she was tied by "golden handcuffs", but the BBC paid a six-figure sum to free her.
Mr Birt offered her the lesser post as head of radio, in which she had no experience. Yet despite that disappointment, she was still being tipped as the next director-general.
When she arrived, opinion at Broadcasting House took against her because she was a Birt appointee who hailed, like him, from commercial television. To beleaguered broadcasters, she personified the enemy - "Birt in a skirt", in a phrase coined by the Independent.
When she sent an excited memo to the staff saying she was "knocked out by the treasure house" of BBC Radio, they responded sourly by leaking the memo to the press and accusing her of being ignorant and patronising. Nor was Ms Forgan a great hit with radio listeners, and she was stung by the ferocity of attacks on changes made to familiar programmes.
She had her successes, though. One was to persuade the governors to invest pounds 10m in digital radio, due on the air in 1998. It will provide, for anyone who buys a new radio set, perfect reception of all five existing BBC channels and allow space for extra ones - for example for cricket commentaries.
Yet this gave rise to more worries. Some of the extra channels will be used for subscription services, to top up licence fee revenue. Ms Forgan, committed to public service broadcasting untainted by profit, fears that Sir Christopher Bland, an entrepreneur, will want to spread the commercial ethic further than she would like.Reuse content