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Star Wars actress Carrie Fisher is in intensive care after suffering a "massive heart attack" while flying from London to Los Angeles.

Passengers on board the plane performed CPR on the "unresponsive" 60-year-old star after she went into cardiac arrest as she flew home just days before Christmas.

Fisher was rushed to a nearby hospital after the United Airlines plane declared a medical emergency and landed at Los Angeles International Airport (LAX) shortly after noon local time.

Paramedics administered CPR for 15 minutes before they were able to get a pulse, and the actress was said to be on a ventilator at UCLA Medical Center, TMZ reported.

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The actress was in critical condition earlier in the day, an emergency official told Variety on Friday afternoon.

She was moved from the emergency room to the Intensive Care Unit hours ago, her brother, Todd Fisher, said on Friday evening.

The actor and filmmaker said his sister is not in stable condition.

"She's in the ICU and everybody's praying for her," he told Variety in a phone interview. "There's nothing new from the doctors. There's nothing new at all. ... There's no good news or bad news."

(Image: Getty Images)

(Image: Getty Images Europe)

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Witnesses at the airport told TMZ that Fisher's eyes were closed, she appeared unconscious and she had an oxygen mask on her face as paramedics rushed her through the terminal.

A source told the Los Angeles Times that the mother-of-one was "in a lot of distress" on the plane.

A doctor and nurses who happened to be on board the 11-hour flight from Heathrow Airport administered CPR until medics took over.

Actress Anna Akana was on board the flight when Fisher went into medical distress and she described the upsetting scene in a series of tweets.

She said Fisher stopped breathing and was helped by a doctor and nurse who happened to be on board.

Akana, 27, wrote: "Don't know how else to process this but Carrie Fisher stopped breathing on the flight home. Hope she's gonna be OK.

"So many thanks to the United flight crew who jumped into action, and the awesome doctor and nurse passengers who helped."

Akana wrote that Fisher wasn't breathing "for 10 minutes or so".

CPR was administered by passengers until the plane landed and medics boarded.

Fisher - who was reportedly travelling with her French bulldog named Gary - was "aggressively treated" by paramedics as she was rushed to hospital.

Guitarist Ryan Mendez of the band Yellowcard was on the flight and wrote on Twitter that Fisher went into distress "on the other side of the wall from me".

He said he had no idea the person in distress was Fisher until after the plane landed.

When the medical emergency was declared on board Fisher was reportedly referred to as "incapacitated" in radio communications between the flight crew and air traffic control.

About five minutes before landing the United Airlines pilot told the control tower that the crew had requested medical personnel to meet the Boeing 777-200 at the gate.

He said passengers who were nurses were attending to an “unresponsive” passenger.

(Image: Getty)

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In an audio recording posted online, the pilot said: "We have some passengers, nurses assisting the passenger. We have an unresponsive passenger.

"They're working on her right now."

A United Airlines spokesperson said: "Medical personnel met United flight 935 from London to Los Angeles upon arrival today after the crew reported that a passenger was unresponsive.

"Our thoughts are with our customer at this time and any requests for additional information should be directed to local authorities."

Airport police officers responded to Terminal 7 after a woman was reported in cardiac arrest.

When officers arrived they found paramedics performing CPR on the woman, Officer Alicia Hernandez told the Los Angeles Times.

A Los Angeles Fire Department spokesperson said in a statement: "At 12:11 PM the Los Angeles Fire Department responded to LAX International Airport, Gate 74, for a patient on an inbound flight in Cardiac Arrest.

"LAFD Firefighter Paramedics were standing by for the plane's arrival, provided Advanced Life Support and aggressively treated and transported the patient to a local hospital."

(Image: Getty)

Fisher has been travelling the world to promote her new autobiography, the Princess Diarist, and posted a number of photos during her visit to the UK and other countries in Europe this month.

She filmed episodes for the TV programme Catastrophe while in London.

In her most recent tweets she appeared to be in the festive spirit as she posed for photos outside Charles Dickens Lounge at the Eurostar station in Calais, France, and Chocolatier Dumon in Bruges, Belgium.

She also wrote about her plans to watch the franchise's new film, Rogue One: A Star Wars Story.

While in London earlier in the month Fisher recorded an appearance on the Graham Norton Show on December 7 - alongside Grayson Perry, Sandi Toksvig, Nadiya Hussein and Busted - and met fans as she promoted her book at a Waterstones shop on December 11.

The following night she attended a Christmas party and posted a Twitter snap with singer and friend James Blunt.

Fisher was born in Beverly Hills to actress Debbie Reynolds and singer Eddie Fisher in 1956.

She became a global star after portraying Princess Leia in the original Star Wars trilogy and recently reprised her role in the space saga's two most recent films.

Other notable film roles include The Blues Brothers and When Harry Met Sally.

She is due to appear in Star Wars: Episode VIII when it opens in cinemas in December 2017.

Fisher was briefly married to musician Paul Simon in the 1980s and later married and divorced Hollywood talent agent Bryan Lourd in the early 1990s.

She and Lourd have one daughter, Billie Lourd, 24, who is also an actress.

Over the years she has openly discussed being diagnosed with bipolar disorder and her struggle with drugs alcohol.

Devastated fans expressed their shock and sadness on social media, and offered prayers and thoughts for the actress.

Star Wars co-star Mark Hamill led tributes online from fellow actors, writing on Twitter: "as if 2016 couldn't get any worse... sending all our love to @carrieffisher".

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Actor William Shatner wrote: "I ask everyone to stop for a moment and send special thoughts to @carriefisher."

British actor Peter Mayhew, who portrays Chewbacca in the franchise, tweeted: "Thoughts and prayers for our friend and everyone's favorite princess right now."

Fellow British actor Gwendoline Christie, known for her roles in Game of Thrones and Star Wars: The Force Awakens, wrote: "The whole world is sending you so much love! Sending you the universes most powerful Force."