It's unclear just how low the new emissions standards may be, although AE references London's mayor, Boris Johnson, and his drive to make every London taxi emissions-free by 2018 as a particular thorn for the Japanese automaker.
It's worth noting that the NV200 taxi is both cleaner and more fuel efficient than London's current fleet of iconic black cabs. That alone makes it seem like reason enough to get the new cabs on the streets. But it's the strictness of the proposed standards and the apparently set-in-stone nature of the NV200's current engineering that is holding up the cab's future.
While Nissan offers an emissions-free e-NV200, it would reportedly need to completely reengineer the NV200-based cab to meet the future standards if they're approved.
Whether that will happen, though, remains an open question.