Miriam O'Reilly, 53, was made redundant from the BBC One programme last April, along with three female colleagues of similar age. They were replaced by the younger presenter Julia Bradbury.
"This was discrimination, which wouldn't happen if I was black or disabled," said Miss O'Reilly, an award-winning journalist who also worked on Radio 4's Farming Today programme.
"To be condemned and made redundant, to have my life cancelled because of my age... No, I'm sorry, they can't do that. That's wrong. The BBC has to start listening. We are the disappeared. No woman is safe in TV once she reaches the age of 45."
Miss O'Reilly said her age was no bar to doing the job. "I have got all my faculties. I run, hike, climb - I love being outdoors. My age shouldn't be an issue if I can still do my job, which I believe I can.
"I was good at what I did. I loved working for Countryfile and was passionate about what we were doing on the show."
However, Miss O'Reilly claims that she was made to feel embarrassed about her age. Senior editors would "get very upset" if her grey roots showed through, she claimed, adding that a cameraman once presented her with a can of hair dye and a colleague suggested she try Botox.
On another occasion, she alleges, a director joked that the advent of high definition television would be a critical moment in her career because "we won't be able to disguise your wrinkles then". Another presenter was told to lose weight, she claimed.
Countryfile's line-up was changed when the BBC announced plans to move the show from Sunday mornings to a primetime slot. While the female presenters were replaced, 69-year-old John Craven remained in his job.
Ratings have gone up since the primetime move, but Miss O'Reilly told the Mail on Sunday: "They justify all this with audience figures, but how do you know you wouldn't get those same figures with older women? There are no older women in primetime. Surely that needs to change."
She is due to face her former employers at a tribunal in November. It emerged last week that the number of sex and age discrimination brought against the BBC by female employees has quadrupled in the past two years.
The corporation faced criticism for sacking Moira Stuart, the newsreader, when she was 55, and replacing Strictly Come Dancing judge Arlene Phillips, 67, with Alesha Dixon, who is half her age.
Newscaster Anna Ford and Gardener's World presenter Carol Klein have also spoken publicly about perceived ageism within the corporation. Other broadcasters have faced accusations of discrimination against older women - Selina Scott won a £250,000 settlement against Channel Five in 2008, claiming she was passed over for a job due to her age.
Mariella Frostrup, the Radio 4 presenter, recently claimed the Today programme lacked women presenters because it was run by "a bunch of misogynists", although she later backtracked and said her remark had no factual basis.
When Miss Bradbury was unveiled as the new face of Countryfile, her age was given at 36. However, it later emerged that she was actually 38.
A BBC spokesman said: "Our programmes always strive to reflect as broad a range of diversity as possible. For example, Kirsty Wark, Maxine Mawhinney and Martha Kearney regularly feature on BBC television and radio shows as well as presenters including Gloria Hunniford, Sheila Hancock, Jennie Bond, Angela Rippon and Annie Nightingale.
"Any suggestion that the presenters of ‘Countryfile’ were replaced on the grounds of age is absolute nonsense.”