The Home Secretary's postal ballot was examined by a judge during a probe into an infamous vote-rigging scandal, it emerged on Sunday night.
Shabana Mahmood denied any suggestion she had been involved in the widespread corruption during the 2004 local elections in Birmingham.
Copies of Ms Mahmood's ballot application and her declaration of identity document were made public for the first time over the weekend and show she used decidedly different signatures.
Ms Mahmood's father, Mahmood Ahmed, was one of Labour's election agents in Birmingham at the time and was briefly under suspicion of being involved in the scandal amid unfounded claims he filled in postal ballots for family members.
The case against him was thrown out and Ms Mahmood was never directly accused of being complicit.
A spokesman for the Home Secretary on Sunday night strongly denied any suggestion of wrongdoing and said both signatures were indeed hers.
He said any discrepancy was down to Ms Mahmood experimenting with different styles. He said: 'Shabana signed both of these documents. As a student, aged 22, she signed two different documents, on different occasions, in different ways.
'That does not change the fact that the writing is hers.' The spokesman said the two documents were filled in several weeks apart.
Shabana Mahmood denied any suggestion she had been involved in the widespread corruption during the 2004 local elections in Birmingham (file image)
Copies of Ms Mahmood's ballot application and her declaration of identity document were made public for the first time this weekend and show she used decidedly different signatures
He said the allegations against her father, who is no longer involved in local politics, 'were tested and dismissed, with no adverse finding, over 20 years ago'.
He added: 'Following the testimony of a court-appointed handwriting expert, as well as witness statements, allegations against Mahmood Ahmed were dismissed.'
In 2005, High Court judge Richard Mawrey QC, sitting in a special election court, ruled there had been 'widespread fraud' relating to Labour Party councillors on Birmingham City Council at the previous year's local elections.
It had taken place in communities with a large proportion of Muslim voters, with the judge concluding Labour activists had faked thousands of votes to offset a loss of support because of the Iraq war.
He said the scandal 'would disgrace a banana republic'.
Five Labour councillors from the Bordesley Green and Aston wards were convicted of electoral fraud, required to step down and the ballots re-run.
It has now emerged that Ms Mahmood's postal ballot was initially among the vast number of documents examined in the trial, Birmingham newsletter The Dispatch revealed.
The then-Labour government introduced legal changes requiring postal vote applicants to supply their signature and date of birth.
It comes ahead of this week's local elections as experts predict Labour could lose thousands of councillors, including control of Birmingham council.