One Direction singer Niall Horan has sparked speculation that he is dating fellow pop star Ariana Grande as the pair were spotted hanging out after her concert at The O2 Arena on 1 June.

Following her packed out show on the first UK date for her The Honeymoon world tour, the One Last Time hitmaker is said to have changed into a revealing crop top and pencil skirt to hit London's burlesque club Cirque Le Soir with her entourage.

Horan, who looked causal in skinny jeans and brown boots, later joined the group at the venue.

Grande sent the rumour mill into overdrive by posting a video on Instagram showing the pair and other friends messing around with helium gas. She added the caption: "new friends + old friends + helium balloons @lexie1225 @alfredoflores @niallhoran @chantelgonsalves".

Bookmaker Paddy Power is offering odds of 5/1 that the pair will confirm their romance by the end of 2015.

"With Niall's eye for the ladies he could have 'Grande' plans for Ariana," a spokesperson told Unreality TV.

If the rumours are true, Grande may have a rival for Horan's affection.

The 21-year-old heartthrob has been romantically linked to Australian model Melissa Whitelaw since February, when she was seen in Sydney leaving a flight right behind the Irish hunk during One Direction's On The Road Again Tour.

After receiving death threats from devoted Directioners, she jumped on Twitter to tell fans: "I haven't done anything to any of you so please stop sending me mean comments. For all you asking, yeah Niall and I dating. Niall has not posted about us because we have decided we will wait until it's the right time."

In April, he appeared to go public with their relationship at Great Gatsby Ball in London, an event organised to support cancer charity Trekstock. Although he arrived on his own he left the bash with Whitelaw, who looked stunning in a black floor-length gown.

Grande is back in the market after calling time on her eight-month romance with rapper Big Sean, citing "conflicting tour schedules".

"They both deeply care for each other and remain close friends," their publicists said in a joint statement back in April.