"seriously threatening" is correct and can be used in written English.
You can use it to talk about something that is causing a real danger or risk to someone or something. For example: "The storm was seriously threatening our safety."
The Swiss government feared that more indictments could follow, seriously threatening the existence of some banks.
ANNALS OF CHEMISTRY about aerosol sprays, which may be seriously threatening the environment.
In September violence erupted in the West Bank and Gaza, seriously threatening the peace talks.
After the break both sides enjoyed good spells of possession without seriously threatening to score.
He believed that Friedman got involved because "Kellner's behavior was seriously threatening to the community's power structure".
The goal did little to alter the flow, with Hull enjoying the bulk of possession without seriously threatening a second.
As a result, some of the world's most productive agricultural soils are now being affected by plastic pollution, seriously threatening soil health and food security.
If true, however, the real trick will be to utilize these appraisals without seriously threatening levels of supervisory authority.
That leaves Standard and Poor's as the only credit-rating firm seriously threatening the United States with a downgrade.
Origi put himself about but, at 2-0 down, Liverpool lost all confidence in possession, never seriously threatening a comeback.
Tells how Dr. Thomas Corbett, an anesthesiologist at the University of Michigan, became concerned about… ANNALS OF CHEMISTRY about aerosol sprays, which may be seriously threatening the environment.
