Militant suffragettes threatened to shoot Liberal Prime Minister Herbert Asquith, newly released National Archives files have revealed.
Women's vote campaigners picketed the House of Commons' carriage entrance for eight weeks in September 1909.
The government was informed by a member of the Women's Freedom League that militants had been honing their accuracy at pistol shooting.
Two suffragettes were reported to have been seen at a shooting range.
Police never traced the women, but the Home Office was concerned about the pickets.
In the age before blanket security for senior politicians, the women were able to get close to ministers to register their protests.
Officials considered moving the protests away from Parliament for the safety of MPs.
A Mrs Moore of East Dulwich, described as a moderate, contacted the Metropolitan Police with a letter from a militant suffragette containing the threat to shoot Asquith.
Mrs Moore told officers she thought the situation was "getting out of hand".
Conspiracy to murder
The two suspected assassins had been using the same shooting range as Indian assassin Madar Lal Dhingra, who had recently killed Sir William Curzon-Wylie, aide to the secretary of state for India, and an onlooker.
The archive file says: "We have in fact prima facie grounds for believing, though of course not evidence, that there is something nearly amounting to a conspiracy to murder."
Security around politicians was often light
Unwillingness to publicise the threat against Asquith led to plans to move the protest being dropped.
"The prominence which would be given to this in the press would probably act in the minds of these half insane women, and might suggest effectively the commission of the very act which we wish to prevent.
"Moreover, the removal of the pickets would be looked on by them as an act of violence and injustice, and would make them furious and more ready to commit such a crime."