At once and In haste

At once adverb - Without delay or hesitation; with no time intervening.

In haste and at once are semantically related. In some cases you can use "In haste" instead an adverb phrase "At once".

In haste adverb - In a hurried or hasty manner.

At once and in haste are semantically related. Sometimes you can use "At once" instead an adverb phrase "In haste".