Spencer Perceval is said to have uttered: "Oh, I have been murdered" after he was gunned down by John Bellingham on May 11, 1812, in the lobby of the House of Commons while on his way to attend an inquiry into the recent Luddite riots.
Four patterned floor tiles in Parliament, thought to mark where Perceval fell, were removed by workmen during recent renovations, leaving the historic event unmarked in the Palace of Westminster.
In another quirk of parliamentary history, North West Norfolk MP Henry Bellingham is a descendant of the assassin.
And yesterday House of Commons Speaker John Bercow insisted no grudges were held against Mr Bellingham as he helped unveil a new plaque in St Stephen's Hall, near to the location thought to mark the spot of the assassination.
Mr Bercow also joked it would be wrong to suggest the wicked behaviour of John Bellingham had been passed on to Henry.
The plaque was installed following efforts by Northampton North MP Michael Ellis, as Perceval was previously an MP for Northampton.
Northampton South Tory Brian Binley joined Mr Ellis, Mr Bellingham and parliamentary officials to unveil the new plaque. Mr Ellis said the new memorial was a "fitting tribute" and long overdue.
He said: "People know more about the assassination of (US presidents) John F Kennedy and Abraham Lincoln than they do about our own prime minister - maybe because it was so long ago, perhaps because he was short-lived as prime minister but it's right near where he fell we do mark that occasion."
Mr Ellis added: "When my constituents visit Parliament from Northampton, they want to see this spot and they are fascinated by this being the spot of an assassination of a former MP for the town that I now represent."