  We experimentally study the electronic spin transport in hBN encapsulated single layer graphene nonlocal spin valves. The use of top and bottom gates allows us to control the carrier density and the electric field independently. The spin relaxation times in our devices range up to 2 ns with spin relaxation lengths exceeding 12 $\mu$m even at room temperature. We obtain that the ratio of the spin relaxation time for spins pointing out-of-plane to spins in-plane is $\tau_{\bot} / \tau_{||} \approx$ 0.75 for zero applied perpendicular electric field. By tuning the electric field this anisotropy changes to $\approx$0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane Rashba spin-orbit coupling. 
Controlling spin relaxation in hexagonal BN-encapsulated graphene with a   transverse electric field