Flight Crew Air Transport Pilot (ATP)
Confusion; Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
At FL340; in radar contact with Shannon; another aircraft was in radar also 10 miles North of us at FL370. Shannon asked if we could climb to FL380. We were able and told Shannon so. Shannon responded with maintain FL380. My First Officer responded 'climb and maintain FL380.' I started a climb and as we passed FL345 Shannon asked if we were climbing saying we had no clearance; but in the same transmission said climb and maintain FL380 there was no conflict. We continued the climb to FL380. The remainder of the flight was normal.
Upon return from my rest break; approximative 1 hour before landing; I was told by the Captain that while in Shannon Center sector; under radar contact from Shannon ATC; was a radio communication misunderstanding between the flying crew and the ATC controller about an altitude clearance; misunderstanding solved without any further issues.
Air carrier flight crew reported climbing without a clearance.