Katy Perry took her grandmother to Las Vegas to celebrate her 90th birthday.

The 'Firework' singer and her new husband, British comedian Russell Brand, took her father Keith's mother Ann to the Planet Hollywood Resort and Casino in the famous US gambling town, where they celebrated with a meal at Strip House steak restaurant.

Katy - whose real name is Katy Hudson - tweeted: "Happy 90th birthday grandma HUDSON! You're the reason why!"

Katy also had another reason to celebrate while she was in glitzy city - as this week it was revealed that her song 'California Gurls' was the biggest selling digital song of 2010, after it was downloaded four million times in the US and sold eight million singles, digital tracks, mobile tracks and videos worldwide.

Katy, 26, is close to her family and has even previously revealed that she like to borrow her grandmother's clothes.

She said: "It doesn't matter who makes it, where it comes from, whether it's your grandmother's closet or it's a couture showroom. If it looks good on you, wear it!"