Immediately up on can be replaced with several synonyms to convey the same sense of urgency or promptness. Words like promptly, right away, instantly, without delay, and forthwith are some suggested alternatives. All of these words imply the need for immediate action, suggesting that something requires attention or action without any delay. All of these words emphasise a sense of urgency, conveying a clear message that immediate action is required. It's worth remembering that the choice of synonym may depend on the context in which the phrase is being used. As such, it is essential to be certain of the precise context and meaning needed to choose the appropriate synonym.