Can single engine aircraft fly in class A airspace? A friend surprised me the other day by saying that single engined aircraft are not allowed to fly in class A airspace. 

I understand that the aircraft is under IFR in class A, with ATC clearance, (flight plan) and must meet PBN specifications, however I can’t find any documentation in my textbooks, EASA or ICAO stating that single engined aircraft are not allowed to fly in class A. 

If anyone could resolve this issue it would be much appreciated.
Single-Engine aircraft can fly in Class A airspace. Pressurization or oxygen masks are required above 18,000 feet (no cannulas).