  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.
