LA LÍNEA DE LA CONCEPCIÓN, Spain — Fears of disruptions following Britain’s departure from the European Union were replaced by coronavirus-related restrictions on border traffic between Spain and Gibraltar on Monday, the first working day at the United Kingdom’s only land border with the European mainland.

Only a share of essential workers from an average of 15,000 who cross the fence between Spain’s La Línea de la Concepción and the British territory on a normal day were venturing into Gibraltar, which went into lockdown late Saturday amid a surge in virus cases that is putting under pressure its limited health infrastructure.

Under the new stay-at-home order, the 30,000 residents on the British speck of land on Spain’s southern tip are only permitted to venture out for work, exercise, medical appointments or to buy essential items. Gibraltar authorities have reported more than 1,300 new cases during the last month, more than double from the levels in early December, and are investigating if the surge is linked to the new virus variant that has rapidly spread in Britain.