The U.S. toymaker, however, said it still expects to close the deal no later than the first quarter of 2020.
The UK's Competition and Markets Authority (CMA) had earlier said it will consider whether the deal could hurt competition and invited comments on the acquisition. (reut.rs/2rX11Av)
In an email to Reuters, Hasbro said that it had been in talks with the CMA about the review, which could last up to 40 business days.
Entertainment One declined to comment.
Hasbro, known for its Nerf guns and Power Rangers action figures among others, made its cash offer in August.
If concluded, the deal is expected to give Hasbro more exposure in its content media push by providing access to popular TV shows such as Peppa Pig and PJ Masks from the independent studio.
Hasbro has been buying smaller companies and tying up with film studios to boost sales of toys linked to movie franchises.
Reporting by Shanima A and Pushkala Aripaka in Bengaluru; Editing by Shounak Dasgupta and James Drummond