The 33-year-old Briton - who was jeered by fans during the bout - landed nothing of note until a huge right to the jaw ended Helenius' night.

The win - Joshua's first stoppage victory in two-and-a-half years - sets up a blockbuster fight with American Deontay Wilder in January.

"It's a fickle sport, you've got to be real about this industry and not get caught up. I've done my job tonight," Joshua told BBC Radio 5 live.

When asked in the ring about the potential fight with Wilder, Joshua joked: "My back's gone, is there a doctor in here? I want to carry this heavyweight division to the top."

After the knockout, an emotionally charged and smiling Joshua climbed out of the ring, high-fived fans and shared a beer with Irish mixed martial arts star Conor McGregor.

Helenius needed oxygen after the heavy knockdown, but recovered and left the ring unassisted after congratulating Joshua.

Joshua extends his record to 26 wins - with 23 stoppages - and three defeats.

Finland's Helenius - who took the bout on a week's notice after Briton Dillian Whyte failed a voluntary drug test - loses his fifth pro fight in 36 outings.

Joshua struggled to find any rhythm but the manner of the finish may just be the confidence-boosting win he so desperately needed, and a gentle reminder to other heavyweights he is not yet done.

Helenius made his way to the ring in a packed out arena at the late time of 23:10 BST. Fans were offered a full refund when Whyte was withdrawn from the card, but such is the draw of Joshua there were no empty blue seats once the main event started.

The Briton confidently strode to the ring to a medley of a violinist playing the title song from the film 'The Godfather' and then the more upbeat 'Insomnia' by Faithless.

The 2012 Olympic gold medallist - fighting at the O2 Arena for the ninth time - kept his eyes firmly locked on Helenius during the introductions, who mockingly clapping back.

Helenius took the middle of the ring and swung a wild left in the first few seconds. The 'Nordic Nightmare' looked unfazed by the hostile atmosphere but neither man landed anything of note in a cagey opener.

Joshua was moving freely, looking to set traps but throwing single shots and not imposing himself on the stand-in fighter.

Fans started to become restless as early as the third. Boos echoed around the arena. The crowd wanted to see combinations, not this tentative approach.

Joshua had not knocked out an opponent in the first half of a fight since beating Eric Molina in 2016, a staggering statistic for someone once considered one of the heaviest hitters in boxing.

The Watford-born fighter landed a solid left in the fourth which sent Helenius backpedalling, but there was no sustained attack to follow.

Helenius grew in confidence, landing jabs to mark Joshua under the eye in the fifth. There were more jeers from a bored crowd at the halfway stage.

"It's hard to find the right hand," Joshua told trainer Derrick James. The American replied: "Keep trying."

Joshua adhered to the instructions. Those fans who left their seats missed what promoter Eddie Hearn described as the "knockout of the year".

A double feint followed by a right sent the Finn to the canvas, with referee Victor Loughlin halting the contest.