First issued in June 1215, the document -- considered one of the most important in the world -- was the first to put into writing the principle that the king and his government were not above the law.

The Magna Carta, or Great Charter, is considered an essential precursor for modern democracy, justice and the rule of law and has formed the basis of legal systems across the world -- and human rights conventions.

Just Stop Oil said Anglican priest Sue Parfitt, 82, and retired teacher Judy Bruce, 85, damaged the glass case protecting the parchment document at around 10:40 am (0940 GMT) at the British Library.

The pair then glued themselves to the glass holding a sign which read: "The government is breaking the law."

AFP contacted London's Metropolitan Police and the British Library but there was no immediate response about the extent of the damage.

In a statement released by the climate action group, Parfitt said Magna Carta was "rightly revered, being of great importance to our history, to our freedoms and to our laws".

But, she added, there would be "no freedom, no lawfulness, no rights, if we allow climate breakdown to become the catastrophe that is now threatened".

The British Library holds two of four surviving copies of Magna Carta.

Just Stop Oil wants the UK government to end all new oil and gas exploration and has promised not to let up in its protests until it does so.

Its activists have targeted numerous high-profile events with stunts over the past year, including the Wimbledon tennis tournament and British Open golf tournament, as well as art galleries and museums.