In a word, no. Telling a client or partner that there is going to be a change in the personnel they are dealing with is something which must be handled by your employer. This lets them manage the process so that they can ensure the client/partner continues to receive the service they need from your employer; by going in and saying "I'm leaving" or even "I'm thinking about leaving" will naturally lead to the client/partner becoming concerned about the continuity of the business relationship, particularly if you are a critical part of the relationship.
There is no at-will employment in the country in question. E.g. there is no threat of termination.
You may say that; in at least some circumstances, I would regard this as misconduct in which case there would be a threat of termination in almost any jurisdiction. Even if there is not a direct threat of termination, actions like this are going to mean that you're at the top of the list to be made redundant as you're clearly not committed to your current employer.