This, say the website organisers, has been done by both Labour and the Conservatives in Islington South, and also by Labour in Bermondsey and Old Southwark.
According to The Straight Choice site, the law is very clear:
"No person shall for the purpose of promoting or procuring the election of any candidate at a parliamentary election issue any poll card or document so closely resembling an official poll card as to be calculated to deceive."
BBC Source (the links I put in link to the leaflets in question).
More proof, if it were needed, that both main parties fight as dirty as one another. Can't say it makes me overly excited about my vote :/