Roche Holding AG's pending takeover of Spark Therapeutics Inc. has finally received approval in the U.K. after months of delays.

The U.K. Competition and Markets Authority granted unconditional clearance to Swiss drugmaker Roche's acquisition of Spark, a deal worth about $4.3 billion in cash.

In March, Roche launched a buyout offer to acquire all outstanding shares of the Philadelphia-based gene-therapy maker for $114.50 apiece. The offer has been extended several times to allow Britain's antitrust regulator and the U.S. Federal Trade Commission to look further into the deal.

After 10 extensions, the offer is set to expire at 5 p.m. ET on Dec. 16, unless further extended.

Deal closing is still subject to certain conditions, including expiration or termination of the waiting period under the Hart-Scott-Rodino Act, and the tender of the majority of Spark's shares.

MacKenzie Partners Inc. is information agent for the tender offer.