The Slough-headquartered company, which owns Durex, agreed to buy K-Y from Johnson & Johnson in March 2014.
K-Y and Durex hold almost three-quarters of the market share in supermarkets and national pharmacies.
Therefore, the deal could lead to higher prices, says the Competition and Markets Authority (CMA).
"Consumers and retailers differentiate between these two products to some extent," said CMA inquiry chair Phil Evans.
However, the overlap in the market meant that the acquisition could reduce competition and raise prices, he added.
CMA is inviting responses to their findings. It will publish its final decision by 18 August.
K-Y started as a prescription product in 1917 and switched to over-the-counter in 1980.
It has a "very high trust score amongst women", says Reckitt Benckiser.
Reckitt Benckiser's possible acquisition of the K-Y brand "could lead to higher prices for personal lubricants", says a UK regulator.