Roche wants to buy U.S.-based Spark to gain a foothold in gene therapy as well as add to its portfolio against hemophilia A, where the Basel-based company already has the treatment Hemlibra that is due to surpass $1 billion sales this year.

Competition authorities have been scrutinising the deal to ensure a Roche gene therapy-Hemlibra combination in the lucrative rare disease market would not give it an unfair advantage over rivals.

Britain's competition agency came to the conclusion it would not, even as the U.S. Federal Trade Commission (FTC) has yet to formally weigh in. The CMA said it had "cooperated closely" with the FTC.

"The CMA found that Spark is not the only supplier developing a gene therapy treatment and that its products are not currently considered to hold any particular clinical or commercial advantages over those being developed by other suppliers," the CMA said.

"The CMA therefore found that the deal between Roche and Spark would not negatively affect competition," the CMA said in a statement issued by the London Stock Exchange.

The competition review has dragged on for months since the deal was announced in February, with delays taking Roche Chief Executive Severin Schwan off guard as he thought the transaction would be completed more quickly.

Schwan is still hoping for the deal to be completed before the end of 2019.

By John Miller