European patent or Unitary Patent proprietors can only have their rights re‑established if they can show that they acted with all due care required by the circumstances in their attempt to observe the missed time limit. All due care means all reasonable care, which is the standard of care that the notional reasonable competent proprietor or their representative would employ in all relevant circumstances. The proprietor will normally have to prove that the failure to meet the time limit results either from exceptional circumstances or from an isolated mistake within a normally satisfactory monitoring system.
Even when the existence of exceptional circumstances is proven or when non-compliance with the time limit can be traced back to an isolated mistake, all due care must have been taken by all the parties involved (e.g. proprietor, representative, assistants, etc.). This requirement is fulfilled, for example, when an assistant makes an error despite having been carefully selected, instructed and supervised by the representative.
Refer to the EPC Guidelines for further information on the application of the due care criteria at the EPO (see EPC Guidelines, E‑VIII, 3.2).