Barack Obama has issued a supportive message to John McCain in the wake of news the US Senator has been diagnosed with brain cancer.

The Mayo Clinic Hospital said the tumour was found after the former presidential candidate, who Obama defeated in the 2008 election, underwent a minor procedure last week to remove a blood clot from above his left eye.

The 80-year-old has now been diagnosed with glioblastoma - a particularly aggressive type of brain tumour which around 20,000 Americans are diagnosed with each year. The American Cancer Society considers the five-year survival rate for patients over 55 to be around four per cent.

McCain and his family are considering further treatment options including a combination of chemotherapy and radiation.

Heaping praise on the Republican Senator of Arizona who is a Vietnam War veteran, former President Obama hailed him as one of the most courageous fighters he had ever met.

“John McCain is an American hero and one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John,” he said.

There has been a public outpouring of grief since McCain’s illness was announced on Wednesday. President Donald Trump, Bill Clinton and George H.W. Bush have put their political tensions behind them to express their support and condolences for McCain.

Senate Armed Services Committee Chairman Senator John McCain speaks on Capitol Hill in Washington, DC (AP Photo/Jacquelyn Martin)

“Senator John McCain has always been a fighter,” President Trump said of the politican he once blasted for having been “captured” in Vietnam. “Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. Get well soon.”

Hillary Clinton said: “John McCain is as tough as they come. Thinking of John, Cindy, their wonderful children, and their whole family tonight.”

Bill Clinton added: “As he’s shown his entire life, don’t bet against John McCain. Best wishes to him for a swift recovery.”

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McCain is famed for his independent spirit and has carved out a reputation for bridging political gulfs and working closely with Democrats on issues. But he is also a strong supporter of US military action overseas and was an outspoken and vociferous critic of Obama’s foreign policy.

However, Obama is by no means the only President he has criticised. He has also been repeatedly critical of President Trump since his inauguration.

McCain's daughter Meghan McCain expressed her sadness in a statement on Twitter, saying: "The news of my father’s illness has affected every one of us in the McCain family.”