Selina Scott is set to sue Channel Five, claiming she was lined up as maternity cover for Natasha Kaplinsky on Five News but was discriminated against for being too old.
Scott, 57, is understood to be seeking compensation under age discrimination law.
She will claim Five went back on an agreement that she would cover for Kaplinsky when she goes on maternity leave later this month.
The channel announced in June that Kaplinsky would be replaced by Isla Traquair, 28, and Matt Barbet, 32.
A Channel Five spokesman said: "We do not accept this claim and will be vigorously defending it."
The case is understood to have been listed at the London Employment Service in Holborn, with the Five director of programmes, Ben Gale, named as a defendant.
Scott, who joined ITV's News at Ten in 1980 and helped launch the BBC's Breakfast Time programme in 1983, hit out at ageism in television news earlier this year.
"How many women are there on mainstream current affairs programmes who are over 50?" she said.
"Anna Ford has retired, Moira Stuart has been bumped, yet you look around and see lots of men."
Scott's agent, Knight Ayton Management, was unavailable for comment.
Five brought in another veteran newsreader, John Suchet, when Kaplinsky's predecessor Kirsty Young went on maternity leave in 2006.
Kaplinsky, 35, joined Five News in February this year, presenting its 5pm and 7pm news bulletins.
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