In a statement on Tuesday, Aljazeera said it "maintains a set of journalistic practices built on being fair, impartial, and balanced, and as is the standard practice with every story, Aljazeera is going through a due diligence process of verifying the details of the Daily Mirror report".
It added: "Before making any conclusions Aljazeera needs to be absolutely sure regarding the authenticity of the memo and would hope for a confirmation from Downing Street as soon as possible.
"If the report is correct then this would be both shocking and worrisome not only to Aljazeera but to media organisations across the world."
"[If the memo is authentic] it would cast serious doubts in regard to the US administration's version of previous incidents involving Aljazeera's journalists and offices"
The station added that if the leaked memo was authentic, "it would cast serious doubts in regard to the US administration's version of previous incidents involving Aljazeera's journalists and offices.
"It would also constitute a new chapter in the relationship between two of the most powerful governments in the world and media organisations in general.
"We sincerely urge both the White House and Downing Street to challenge the Daily Mirror report and in the event that the memo is found to be accurate it would be incumbent on them to explain their positions on statements regarding the deliberate targeting of journalists and news organisations," Aljazeera said.
In April 2003, an Aljazeera journalist died when its Baghdad office was struck during a US bombing campaign.
In November 2001, Aljazeera's office in Kabul, Afghanistan, was destroyed by a US missile, although no staff were in the office at the time.
US officials said they believed the target was a "terrorist" site and did not know it was Aljazeera's office.