Electronic equipment including mobile phones, Kindles, Ipads etc are banned from count centres.
Sinn Fein's Órfhlaith Begley, SDLP MLA Daniel McCrossan, DUP MLA Thomas Buchanan, the Alliance Party's Stephen Donnelly and the UUP's Chris Smyth are battling it out to become the new MP for West Tyrone.
The by-election was called after Sinn Fein MP Barry McElduff resigned after posing with a loaf of Kingsmill bread on his head on the anniversary of the Kingsmill Massacre in which 10 protestant workmen were shot dead. The polls close at 10pm on Thursday with a resulted expected early on Friday morning.
Sinn Féin Election Agent Barry McColgan branded the claims "spurious and pathetic".
“One of our election workers was in a polling station early this morning reading a Kindle e-reader," he said.
“He had sought and received permission from the electoral staff on duty to use the e-reader and was openly reading an e-book for two hours.
“For the SDLP to subsequently claim that this amounts to electoral fraud is as spurious as it is pathetic.
“In fact, it has been confirmed that one of the SDLP representatives who raised the complaint was not a named polling agent and was therefore breaching electoral law by being in the building.
“If anyone has questions to answer about this incident, it is the SDLP.”
The Deputy Returning Officer at the count centre has confirmed that a Sinn Fein polling agent was using a Kindle in Strabane this morning and was asked to remove it, which they complied with.
SDLP West Tyrone Election Agent Simon Ashleysaid that it is absolutely essential that electoral rules are fully enforced and Sinn Fein should remember that no-one is above the law.
“The Sinn Fein Election Agent Barry McColgan has blatantly got his facts wrong," he said.
“It is absolutely essential that electoral law is fully enforced and Sinn Fein should remember that no-one is above the law. Sinn Fein know that the use of any electronic item is prohibited inside a polling station.
“The SDLP has reported four separate incidents from West Tyrone polling stations today to the Electoral Office, including the use of mobile phones by polling agents. Any member of the public who sees electronic equipment being used inside polling stations including mobile phones, laptops and tablets should report it immediately.
“Sinn Fein should distance itself from Barry’s comments if they believe in fair and democratic elections.”