Both clubs were set to confirm the transfer early this morning before, during a second stage of the forward’s medical, issues emerged which forced West Ham back to the negotiating table
The Hammers finally completed the deal – which is understood to include a £5million loan fee for this season – after a day of intense and delicate negotiations with the Bees.
An agreement to buy for a fee of £25m before a further £5m in add-ons is thought to have been included.
Progress had been exceedingly slow throughout the week, with Benrahma having to settle an issue over loyalty payments with Brentford upon his return from international duty on Wednesday, before heading to begin his medical in east London on Thursday.
The Algerian international is thought to have been personally pushing to ensure the deal was secured, and becomes the third new signing of the window for David Moyes after Vladimir Coufal and Craig Dawson.