Watch Metallica's Kirk Hammett perform 'Doctor Doctor' with his heroes UFO

Returning on stage for the encore, UFO frontman Phil Mogg told the 1,500-strong crowd at the Avalon: “We’re going to do something which is a bit out of our field. We have a guest tonight…”

Kirk Hammett then entered the stage to huge cheers and promptly launched into UFO’s 1974 timeless classic ‘Doctor Doctor’ and 1975’s ‘Shoot Shoot’ alongside the British rock icons.

You can watch fan-filmed footage of opening number ‘Doctor Doctor’ right here:

UFO are currently playing a select number of dates in North America as part of their 50th anniversary Last Orders Tour, which are their final shows with Phil Mogg as frontman.   

Sadly, just days after UFO wrapped up the UK & Ireland leg of the swansong trek with a sold-out homecoming show at London Shepherd’s Bush Empire in April, guitarist and keyboardist Paul Raymond died from a heart attack.

Kirk Hammett was among the plethora of rock stars who paid tribute to Paul, writing on Twitter: "Make more heavy metal in heaven bro!"

In the wake of Paul Raymond’s death, former band member Neil Carter rejoined UFO for their remaining summer European shows and their North America tour in the autumn.

Speaking to Guitar World in 2012 about how UFO’s fourth album ‘Force It’ was a profound influence on him as a teenager, Hammett said: "The first track my friend played was 'Mother Mary', and I thought, wow, these guys are just as heavy as Thin Lizzy, Aerosmith, Montrose and all the other hard rock stuff that I was listening to before I got into heavy metal.

"When it got to the guitar solo, I was just blown away by Michael Schenker's tone, phrasing and technique. By the time the second solo came on with the fastest descending lick I'd ever heard, I was totally hooked! I immediately grabbed the album cover and saw the picture of Schenker playing a Flying V.

"From that point on, I knew there was an entire rock vocabulary out there that was not just based on pentatonic scales, and I set out to learn as many Schenker solos as possible while trying to write heavy riffs just like UFO. I also wanted a Flying V so bad!

"That record taught me a lot about solo structure, phasing and melody, as well as playing for the song. I was amazed how UFO could be so heavy and so melodic in the course of one song. I think the band I was in at the time added two UFO songs to its set that week.

"For me, the standout tracks on 'Force It' are 'Mother Mary', 'Shoot Shoot', 'This Kid's', 'Out On The Street' and 'Let It Roll'. Every time I pick up the guitar and start improvising, I think a lick or two from that album squeaks out subconsciously. I probably spew the most Schenker licks on 'Kill 'Em All' because everything was still kind of new to me at that time."

In 2015, Hammett jammed with ex UFO axeman Michael Schenker on VH1’s That Metal Show.