The announcement of a new Time Limited Zone (TLZ) was made after all UK airspace was reopened yesterday evening.
The CAA said the TLZ - introduced from midday today (local time) would allow flights for a limited time at higher ash densities than are currently allowed.
To operate in the new zone, airlines need to present the CAA with a safety case that includes the agreement of their aircraft and engine manufacturers.
The CAA said this meant that in future some areas of UK airspace that would have previously had to close would be able to remain open.
CAA chief executive Andrew Haines said: "I'm pleased that the huge efforts we're all making across aviation to keep flying safe while minimising the disruption from the volcano have resulted in further progress.
The measures follow heavy criticism of the no-fly zones from airline industry chiefs.
British Airways chief executive Willie Walsh said blanket bans on flying were "a gross over-reaction to a very minor risk".
And Virgin boss Sir Richard Branson described the situation as "beyond a joke".
The announcement came after the UK's National Air Traffic Service confirmed that British airspace was to reopen from 7pm British time on Monday until 1am (local time).
A no-fly zone would remain over parts of the North Sea, affecting some helicopter flights.
The majority of UK airports reopened today with only a few - including some in the Shetland Isles - still closed. (ANI)