Air carrier flight crew reported climbing without a clearance.

Attributes
ACN	1458466
Time
Date	201706
Local Time Of Day	0001-0600
Place
Locale Reference	EISN.ARTCC
State Reference	FO
Environment
Flight Conditions	VMC
Light	Dawn
Aircraft 1
Make Model Name	Widebody Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Climb
Route In Use	Oceanic
Flight Plan	IFR
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

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.

Original NASA ASRS Text

Title: Air carrier flight crew reported climbing without a clearance.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.