Yourself, in person, before the letter goes out, and with the reason attached. Those four points cover nearly everything there is to say, and all four get broken surprisingly often.
Why it matters
The Belgian agency world is small. The account director you part with today will be at another agency in two years, possibly in the pitch you launch then. How you leave decides who takes part and with how much enthusiasm.
There is also a practical reason: you still need several months. A three to six month notice period means these people will work on your brand for another half year. A termination that lands like a slap costs you exactly those months.
How to do it
1. Say it yourself, to the person who invested most. Not by email, not through procurement, not through an assistant. One call or one meeting.
2. Give the real reason. Not at length, but honestly. "We are moving to an agency stronger in e-commerce" is usable; "we are going in a different direction" is an empty sentence everyone recognises.
3. Only then send the formal letter, with the notice period and the dates.
4. Say what you expect during the remaining period, and ask who stays on your account. See what a normal notice period is.
5. Start the handover immediately, not in the final week. See how to make sure the handover goes well.
What not to do
Launch a pitch without saying so. They will hear about it anyway, because the market is small, and then you lose the conversation before it starts.
Leave it to the new party. Your current agency should not hear it from its successor.
Wait until the last day of the notice period. An agency told in time can plan a clean handover; an agency told late can only tidy up.
And ask for a debrief
Ask what they thought went wrong, including what you could have done differently. It is the cheapest advice you will ever get, because it comes from someone who knows your brand and has nothing left to lose.
If a selection runs ahead of the termination, the order of those two is half the work. IKAg guides that process on the advertiser's side: see Agency selection & pitches.
See also: what switching agencies costs and who owns the work when you stop.