John McCain revealed his doctors gave him a 'very poor prognosis' when he first found out about the rare and aggressive brain cancer that he continues to battle.

The Arizona senator said his doctors told him at the time that his condition was 'very, very serious'.

'So I just said, "I understand." Now we're going to do what we can, get the best doctors we can find and do the best we can. And at the same time, celebrate with gratitude a life well-lived,' McCain said during an interview on 60 Minutes, that aired Sunday night.

McCain underwent surgery in July for a brain tumor that was later found to be a form of glioblastoma, an aggressive form of brain cancer that is nearly always fatal.

The Republican senator spoke candidly about the grim diagnosis following a routine doctor's appointment.

John McCain said his doctors have given him a 'very poor prognosis' as he continues his battle against brain cancer

McCain said he was leaving the Mayo Clinic in Phoenix after a routine checkup when he got an urgent call from his doctor.

'You've got to come back,' he recalled the doctor saying.

'Hey, today is Friday. I'll just come in on Monday.' McCain replied.

But the doctor persisted, telling him: 'No, you have to come now. It's very serious.' The doctors had discovered a blood clot above McCain's left eye. 'They thought it was serious enough that they had to act immediately,' he said.

Before the operation to remove the clot, the doctors mentioned glioblastoma, an aggressive form of brain cancer.

But McCain said that the doctors hedged, trying to downplay concerns until tests confirmed the diagnosis.

'Tell it to me straight... I said, "I can take it. Just tell me." And - and then they were more forthcoming,' McCain said.

Lab tests confirmed the diagnosis days later.

'They said that the prognosis is very, very serious. Some say 3 per cent, some say 14 per cent. You know, it's-- it's a very poor prognosis,' recalled McCain.

The cancer that McCain is suffering from is the same type of cancer that took the life of his former Senate colleague Edward M. Kennedy in 2009.

He told CBS' 60 Minutes that he thinks about Kennedy a lot. McCain said Kennedy continued to work despite the diagnosis and 'never gave up because he loved the engagement'.

McCain said he has 'feelings sometimes of fear of what happens,' but counters that with gratitude for having lived 'had a great life'.

McCain was leaving his doctor's office after a routine checkup when he got an urgent call

McCain's wife of 37 years Cindy also spoke out in the new interview, which aired Sunday night

Shortly after the interview aired, McCain's daughter, Meghan McCain posted a heartfelt tribute to her father and others who suffer from glioblastoma

He added: 'it's not that you're leaving, it's that you — that you stayed.'

Shortly after the interview aired, McCain's daughter, Meghan McCain posted a heartfelt tribute to her father and others who suffer from glioblastoma.

'60 minutes came to our ranch and spent time with us in Arizona. I cried the entire time watching my parents interviewed,' she wrote on Instagram.

'Thank you for your respect Leslie Stahl and to CBS for showcasing correctly the inconceivable bravery of both my parents. I still can’t talk about my fathers cancer publicly, but I promise some day I will.

'I pray every morning and night for everyone who suffers from #glioblastoma and the people who love them,' she wrote.

The interview, in which McCain's wife of 37 years Cindy also spoke, comes two days after the senator was the target of a scorching Twitter attack from President Donald Trump.

In a series of messages posted to Twitter, Trump slammed McCain for his lack of support in nullifying the Affordable Care Act, accusing the former Republican presidential nominee of going back on his word.

'John McCain never had any intention of voting for this Bill, which his Governor loves. He campaigned on Repeal & Replace. Let Arizona down!' the President tweeted Saturday morning.

On Thursday, McCain announced that he could not 'in good conscience' support the Republican party's likely last effort to repeal the ACA, a bill sponsored by his close friend Senator Lindsay Graham (R-SC) and Bill Cassidy (R-LA).