A 100ft long blue whale mum and her calf were spotted swimming alongside a tourist boat in California.

Spectacular footage shows the endangered pair gliding in the blue water near a boat off the coast of Newport Beach in Orange County on June 18.

In another clip, one of the tourists could be heard exclaiming in awe as the baby whale flipped its tail in the surface of the water while swimming.

The playful baby then continued to swim around the boat for 45 minutes while its mum was nearby.

Jessica Roame, from Davey's Locker & Newport Landing Whale Watching, said: ‘Amazingly, one of these cow calf pairs began to become curious of the small boat, and its passengers.

‘We call this behavior ‘mugging' because our engines are in ‘neutral' and the whales curiously circle the boat, and can become playful.

‘The baby blue whale in the video below ‘mugged' the boat for over 45 minutes, rolling, spy hopping and enjoying the little boat cruising along next to them.'

Blue whales are the largest animals to have lived on Earth. The longest blue whale on record is a female measuring 110ft and 17inches in the South Atlantic in 1909.

They are classed as endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. The biggest threat to blue whales comes from the commercial fishing industry, through ship strikes and loose nets.