Hailey Baldwin

Justin Bieber serenades Hailey Baldwin outside Buckingham Palace

The pop star performed acoustic versions of Cold Water, his 2016 collaboration with Major Lazer and MO, and Tracy Chapman's 1988 hit Fast Car.

Baldwin watched with fans and proudly filmed on her phone.

The couple did some sightseeing after flying in for Baldwin's catwalk show during London Fashion Week, and were also spotted near the London Eye.

Last week, it was rumoured that Bieber and Baldwin had secretly married after they were seen in a New York City courthouse.