The spoof message was posted in the "campaigning" section of the site
Labour Deputy Leader Harriet Harman is the latest MP to become a victim of internet hacking.
An item was posted on her personal site for several hours announcing her defection to the Conservatives.
It began: "To friends, foes and fans, below is a copy of the resignation letter that landed on Gordon's desk this morning."
Beneath it was a link to a spoof Harriet Harman blog. Ms Harman was not available for comment.
Her "resignation letter" was based on the statement made by MP Quentin Davies when he defected from the Conservatives to Labour last year.
The site, harrietharman.org, appears to have been taken offline following the discovery of the rogue message, which was highlighted by widely-read Westminster gossip blog Guido Fawkes.
In 2006, then environment secretary David Miliband was forced to shut down an experimental wiki site after it was bombarded with surreal and abusive additions.