Spain's embattled Prime Minister Pedro Sánchez is introducing an emergency decree to protect the rights of Britons in Spain and Spaniards in Britain after the UK leaves the European Union.

The decree will be approved by the Spanish Cabinet on March 1, as the clock is ticking both for Spain – which goes to the polls on April 28 – and for the UK, which is due to leave the EU on March 29 with or without a Withdrawal Agreement.

The Spanish government is moving to extend local legal rights to U...