DLG claims it licensed data to McDowall Media last year for one-off use in a consumer telemarketing campaign and an email campaign.
Its solicitors have issued a writ alleging that McDowall Media is in breach of the contract by using some of the data on multiple occasions between October and April this year.
McDowall Media claims the allegations are untrue and is blaming the repeated use of the data on an unidentified client.
Graham Bate, managing director and owner of McDowall Media, said: "Any allegations that we have abused or continued to use data supplied to us by DLG are unfounded and untrue.
"We have taken this matter extremely seriously and placed it in the hands of our solicitors. I am confident the courts will find this to be the case.
"We sold data in good faith to an end user and that end user consequently used the data more than one time. Our lawyer is talking to them."
Bate is a former sales director of DLG and set up McDowall Media in 2008.