Justin Timberlake joined in with a very special one-off reunion of the Backstreet Boys as the second oldest member of the band, JC Chasez, celebrated his 40th birthday on Monday (August 8th).

35 year old Timberlake had his phone on standby to snap a photo of the momentous event, uploaded it to Instagram the following day. He captioned it: “JC’s 40th… And, if you don’t know now you know…”

JC's 40th... And, if you don't know now you know...

A photo posted by Justin Timberlake (@justintimberlake) on

It marked the first time that all five members of the band – Timberlake, Chasez, Lance Bass, Chris Kirkpatrick and Joey Fatone – have been together since their brief reunion at the 2013 MTV Video Music Awards.

Chasez is the second member of the boyband to pass the milestone, with Kirkpatrick having celebrated that moment four years ago, with all the other band members still in their thirties. 44 year old Kirkpatrick shouted out Chasez on Twitter, writing: “Happy birthday @JCChasez! Welcome to the old man club!”

More: Man arrested for slapping Justin Timberlake at golf course during celebrity tournament

A number of other celebs, including Tara Reid, Brody Jenner and 5 Seconds of Summer’s lead singer Luke Hemmings, also joined the party at The Nice Guy bar in West Hollywood.

A source told E! News: “The entire party was casual. The restaurant was rented out for JC's birthday. JC's manager, Eric Podwall, gave the first toast, followed by Justin giving a toast with Joey, Lance and Chris standing next to him while he spoke.”

Of course, the eagle-eyed fans among you will remember that the band briefly reformed back in February 2015 as part of the 40th anniversary celebrations of ‘Saturday Night Live’. No new music has ever been mooted, but the following little information snippet might encourage hard-bitten fans hoping for a permanent reunion.

“Everyone relaxed and ate and drank,” the source continued, adding that the former boy band members “were together at the bar talking towards the end of the night.”

More: ‘Backstreet’s Back, Alright!’ Nick Carter recruits ‘90s pop stars against zombie onslaught in ‘Dead 7’