Watford are now under investigation from the EFL and could face a hefty fine or points deduction.
Chief executive Scott Duxbury said in a statement: "The club had no prior knowledge of any concerns about the authenticity of the document.
"The club takes this matter extremely seriously and has instructed independent solicitors to carry out an investigation into relevant matters and provide the EFL with the solicitor's report early next month."
He added: "The EFL has indicated it is happy with this course of action.
"In light of the ongoing investigation, it would be inappropriate to comment further at this time."
As Watford were a Championship club at the time, the EFL will carry out the investigation, which concerns a proof of funds document provided to the EFL in order to ratify a change in shareholder.