The row between Donald Trump and former US national security officials has intensified, following the US President's decision to revoke former CIA director John Brennan's security clearance.
While senior US officials typically retain their security clearance once they have retired, so they can consult with their successors over key issues when needed.
However, President Trump last week announced his decision to remove Mr Brennan's clearance, accusing him of "erratic conduct and behaviour".
The US President also listed other former officials whose clearance is now being reviewed - many of whom have been openly critical of him, such as former FBI boss James Comey.
Mr Brennan - a frequent and vocal critic of President Trump - claimed the decision to revoke his clearance was part of a "broader effort by Mr Trump to suppress freedom of speech & punish critics".
Speaking on NBC yesterday, the former CIA chief also indicated he would consider going to court to challenge the action and to prevent others having their clearance revoked.
In a pair of tweets today, President Trump addressed the controversy - describing Mr Brennan as "the worst CIA Director in our country's history".
He said he hopes Mr Brennan brings a lawsuit, adding: "It will then be very easy to get all of his records, texts, emails and documents to show not only the poor job he did, but how he was involved with the Mueller Rigged Witch Hunt. He won’t sue!"
Everybody wants to keep their Security Clearance, it’s worth great prestige and big dollars, even board seats, and that is why certain people are coming forward to protect Brennan. It certainly isn’t because of the good job he did! He is a political “hack.”
— Donald J. Trump (@realDonaldTrump) August 20, 2018
Meanwhile, 177 former national security officials - from both Republican and Democratic administrations - expressed their 'strong opposition' to any 'actual or threatened' removal of security clearances.
In a statement, the officials write: "All of us believe it is critical to protect classified information from unauthorized disclosure.
"But we believe equally strongly that former government officials have the right to express their unclassified views on what they see as critical national security issues without fear of being punished for doing so."
The officials add while signatures do not mean they agree with the opinions expressed by Mr Brennan, they do represent a "firm belief that the country will be weakened if there is a political litmus test applied before seasoned experts are allowed to share their views".