Shortly after take-off from Madrid to the Canary Islands, the 737-800 lost cabin pressurisation, which protects against low outside air pressure and lack of oxygen.

"My wife and another woman were bleeding from the ears," passenger Miguel Angel Cayuela told the online edition of Spanish daily El Mundo.

"Nobody was saying anything," he said.

"The cabin crew seemed confused, and when we began the descent we could feel the changes in cabin pressure. Some of us had ear pain, and my wife started to bleed."

A total 12 passengers and four crew were treated by medical services after returning to Madrid-Barajas airport, suffering ear pain or anxiety attacks, said a spokesman for airport operator AENA.

"These are the typical problems in a situation of despressurisation, it is nothing serious," he said.

The Boeing 737-800 plane, on flight FR 2011 to Gran Canaria, returned to the Spanish capital after the loss of pressure, the latest in a string of incidents to hit the budget carrier in Spain.

"I just had the biggest scare of my life....I recommend that you never fly with Rynanair. Masks in the middle of the flight!" one passenger, Daivid Betancor, wrote in a Twitter message.

"My friend's mask did not work and we had to share," another passenger, 31-year-old designer Marcos Aleman, told reporters, according to the online edition of daily El Pais.

Ryanair said pilots carried out standard procedures for depressurisation, deploying oxygen masks and initiating a controlled descent to the proper altitude.

The plane landed back in Madrid at 8:25 am (0625 GMT), it said.

Ryanair said its engineers were inspecting and making necessary repairs to the plane.

"At the same time, Rynair has assigned a replacement plane so that the affected passengers can fly to Gran Canaria," it said. "Ryanair apologises to the 160 passengers affected by this delay."

On Tuesday, another Ryanair flight from Leeds Bradford airport in Britain requested a priority landing at its destination in Lanzarote in the Canary Islands.

The plane, which had to abort an earlier landing attempt because of wind, requested priority because of low fuel concerns but did not declare an emergency, AENA said.

The budget carrier later insisted there were no fuel problems in that incident.

Ryanair is already under investigation in Spain over three flights on July 26 which requested low-fuel emergency landings in Valencia. Storms had prevented them landing at their destination in Madrid and forced them to return home.

Dublin-based Ryanair, Europe's largest budget carrier, operates a fleet of nearly 294 Boeing 737-800s. It transported 8.7 million passengers last month, up from 8.1 million during the same month last year.