Love-struck Justin Bieber serenades Hailey Baldwin in front of Buckingham Palace

His busking days may be over, but that didn't stop Justin Bieber breaking into song outside Buckingham Palace - much to the delight of his partner Hailey Baldwin. 

The rumoured newlyweds were sight-seeing around London when the 'Baby' hitmaker broke out his guitar and reportedly dedicated a song to Baldwin.

According to People magazine, the 24-year-old singer told her he loved her, before breaking into a rendition of his song 'Cold Water'.

Excited fans watched on alongside Baldwin, who was photographed recording the spontaneous recital on her phone.

The 21-year-old model made the trip to the UK for London Fashion Week, but found time to visit some iconic London locations with her loved-up alleged husband.

"They were very affectionate, stopping on street corners to pull each other in close to kiss," an onlooker told E! News.

"They never let each other get too far [away] and were always touching each other and laughing."

Last week, TMZ cameras snapped the pair walking into a courthouse where marriage licences are issued.

Bieber was reportedly emotional at the time, telling Baldwin "I can't wait to marry you, baby."

Newshub.