According to a report in the Sunday Mirror, Scott is seeking compensation under age discrimination law.
The 57-year-old presenter claims Five reneged on an agreement that she would cover for Natasha Kaplinsky when she goes on maternity leave later this month.
But the broadcaster announced in June that Isla Traquair and former BBC London presenter Matt Barbet were to present the flagship 5pm and 7pm bulletins in Kaplinsky's absence.
Scott's lawyers are reported to have lodged the age discrimination claim at the London Employment Tribunal.
Five's new director of programmes, the former BBC commissioning editor Ben Gale, is understood to have been named as a defendant.
Scott, who joined ITV's News At Ten in 1980 and launched the BBC's Breakfast Time programme in 1983, has previously criticised ageism in television news.
In an interview earlier this year, she said: "How many women are there on mainstream current-affairs programmes who are over 50?
"Anna Ford has retired, Moira Stuart has been bumped, yet you look around and see lots of men."
An unnamed source at Five told the Sunday Mirror: "Selina was really keen to do the job, she saw it as a great opportunity to revive her TV career.
"She was furious because she believes she was being dumped merely because of her age."
Scott's agent, Knight Ayton Management, Channel Five and its news supplier, Sky News, were unavailable for comment.
But in a statement to the Sunday Mirror, Five said: "We do not accept this claim and will be vigorously defending it."
Kaplinsky was poached from the BBC to become the new face of a revamped Five News in February. Two months later, she announced she was pregnant with her first child.
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