Mastodon have pulled out of their appearances at Reading and Leeds Festivals next month.
They had been due to headline The Pit but have cancelled due to a "personal family matter".
They've also shelved 14 other dates in Canada and Europe and will be replaced at Reading and Leeds as Pit headliners by Swedish rock act Ghost.
Mastodon will return to the road in South America in the middle of September.
They're due to play Rock in Rio on 25 September followed by shows at Anhembi, Sao Paulo, on 27 September and the Comes Louder Festival on 28 September.
They're also playing Knotfest at San Bernardino, California, on 24 October.
Mumford and Sons, Metallica and The Libertines headline Reading and Leeds Festivals this year on the weekend of 28-30 August.
Mastodon are made up of bassist/singer Troy Sanders, guitarist/singer Brent Hinds, guitarist Bill Kelliher and drummer Brann Dailor.
They toured the UK in November and December last year and played at Sonisphere.
The Atlanta band have released six studio albums with their latest release, 2014's Once More 'Round the Sun, making it to number 10 on the Official Album Chart.
The cancelled dates are:
Montreal, QC, Heavy Montreal Fest - 7 August
Oslo, Norway, Oya Festival - 12
Dinkelsbühl, Germany, Summer Breeze Fest - 14
Hamburg, Germany, Elb-Riot Festival - 15
Dornbirn, Austria, Bregenz Conrad Sohm - 16
Cologne, Germany, Essigfabrik - 18
Berlin, Germany, Kesselhaus - 19
Charleville Mezieres, France, Cabaret Vert - 20
Hasselt, Belgium, Pukkelpop Shelter Marquee - 21
St. Petersburg, Russia, SKK Arena (with Metallica) - 25
Moscow, Russia, Olympysky (with Metallica) - 27
Reading Festival (The Pit Stage) - 29
Leeds Festival (The Pit Stage) - 30
Istanbul, Turkey, 100% Metal Headbangers - 31
Thessaloniki, Greece, Principal Theater Club - 2 September
Athens, Greece, Gazi Hall Entertainment Stage - 3
Another date in Reykjavik, Iceland, on 5 September has been rescheduled for 5 December to coincide with the Rokkjotnar Festival.