The phrase "act at once" is correct and usable in written English.
It can be used to indicate the need for immediate action or response.
Example: "If you notice any issues with the project, please act at once to address them."
Alternatives: "take immediate action" or "respond without delay".
And it could come so soon, so said the Bushies, We had to act at once to save our tushies.
She did not act at once, claiming she thought he was just trying to frighten her; after cutting him down, it was clear he had not been.
In an act at once practical and symbolic, the six women in the group pulled off their mandatory head coverings and used them to start a fire.
In the pilot, Philip learns about this traumatic history and kills the man who, decades earlier, raped his wife; his act at once bonds the pair and nearly wrecks them.
He said Chris Grayling, the transport secretary, should act at once, adding: "All he needs to do is pick up the phone to GTR and rail services can be restored in time for tomorrow's rush hour.
Each carries a copy of what was, statistically, one of the year's most frequent internet downloads, a video of the beheading of an American hostage by Islamic militants, an act at once horrifically real and carefully staged.
Chirac acted at once to reverse many of the Socialists' reforms.
It warned him that unless he acted at once to pay his arrears, his driver's license would be revoked.
But, once asked, my doctors acted at once - as a private patient I don't have to wait for the NHS.
But it is more vital still that it acts at once on the political front, to bring as many as possible of Iraq's Sunni Arabs into the new order.
Here is a movie buff who saw immediately the force in director Marty Scorsese's plea to preserve the libraries of great movies decaying on old film – and acted at once at his Fox while other studio managements equivocated.
