The withdrawal of the United Kingdom from the European Union: no deal
In the event the UK withdraws from the EU without a Withdrawal Agreement, specifically drafted Belgian legislation (see this page) will come into force, which will be based on reciprocity. This legislation is currently being discussed in the Belgian Federal Parliament and the Belgian Parliament will ultimately decide its final content.
Under its current form, this legislative proposal introduces a transitional period, which will last until the end of 2020. During this period, all residence rights are guaranteed until the end of the transitional period for persons who resided legally in Belgium before the date of Brexit. Pending applications for UK international officials and their family members on the date of Brexit will be treated according to the current regulations. International officials who previously held a special identity card and who register with their municipality during the transitional period, will still be able to obtain an E or E+ card. UK international officials who fall under the legislation applicable in Belgium (registration with the municipality) will be encouraged to apply for long term residence as third country nationals in the EU (D-card) during the transitional period to ensure their right to permanent residency (provided they fulfill the necessary conditions). For more details see this page.
UK international officials and their family members who arrive during the transitional period will still be entitled to the special identity card. However, if they wish to register with their municipality of residence, they will be regarded as third country nationals. Only certain family members of UK international officials who resided legally in Belgium before the withdrawal of the UK, or who have a pending application at the moment of withdrawal, will be regarded as family members of an EU national. UK international officials and their family members who arrive after the end of the transitional period will be regarded as third country nationals if they decide to register with their municipality of residence.
For more general information about the procedures and the regulations applicable in Belgium, please refer to your municipality of residence or to the website of the Belgian Immigration Office.