EU TM no deal Brexit
A European trademark registration ("EUTM") covers every member state of the European Union. From 29 March 2019, this may no longer include the UK.
In the case of a "no deal" Brexit, the UK government is planning to pass legislation to automatically give every EUTM holder a free UK trade mark registration ("UKTM"). The fully independent UKTM can be challenged, assigned, licensed or renewed, separately from the original EUTM.
Although it will take time for the legislation to be passed, it will be retrospectively backdated to take effect from Brexit day.
Most existing EUTM holders will notice the difference when the time comes for renewal - they will need to pay extra fees to maintain their UKTM and EUTM, but there are no other steps you have to take before then.
However, if your EUTM is filed, but not yet granted on Brexit day, you will need to file a separate UKTM application. This is likely to affect international applications filed using the Madrid Protocol as well.
Full details of the UK strategy can be found at: https://www.gov.uk/government/publications/changes-to-trade-mark-law-if-the-uk-leaves-the-eu-without-a-deal/changes-to-trade-mark-law-in-the-event-of-no-deal-from-the-european-union