The photograph was published in The Sun today. The newspaper claims Jack was visiting the rival soap's set when he took the image as a joke and tweeted it around 3am before deleting it to avoid fan backlash.
A source said: "He thought it would be funny to pose for this snap and posted it online but took it down within seconds. Jack knew this could get him in trouble. There's big rivalry between the soaps but the ITV bosses don't want any headaches over rows."
Jack, who plays David Platt in Corrie, was not commented.
ITV's annual summer party took place at the Emmerdale set this year, so the stars of Corrie and Emmerdale were mingling together when the picture was taken.
A source said: "This [picture] was obviously just hijinks at the end of a great night."
Reaction on Twitter varied from those unimpressed by the photo and those who backed Jack and said he was clearly just messing around.
Get a sense of humour and leave @JackPShepherd88 alone!its a JOKE— paul brookes (@bruxie999) July 16, 2015
@jackPShepherd your Yorkshire citizenship will be revoked apologise now.— NORTHERNBULL (@davehutchinson5) July 16, 2015