Poundland wants to buy its closest rival in an agreed deal worth Â£47.5m in cash and Â£7.5m in shares.
Initial investigations by the Competition and Markets Authority (CMA) said this could cause issues, including store closures, in 80 areas.
Now Poundland has rejected the CMA's suggestion to re-think the deal.
In its initial findings, the CMA said the deal could worsen the position for shoppers in local areas owing to a cut in quality, fewer promotions and store closures.
It said there was "a realistic prospect of substantial lessening of competition" in these areas where the two companies overlapped.
The authority suggested the concerns could be addressed in a "clear-cut manner" by the companies to prevent a full investigation.
Now Poundland has said there would be no changes to its plan, and has invited the CMA to move to a full review.
The company said it "remained confident that the combination of the two businesses will provide better choice, value and service for 99p Stores' customers".
It also claimed that the deal was "great" for anyone who held a stake in either business.
Discount store Poundland has invited the competition authorities to move to a full review of its proposed tie-up with rival 99p Stores.