CCW officers Kate Lock and Louise Luddington spot the dolphins

Onlookers estimated up to 300 common dolphins gathered near the Skomer Island Marine Nature Reserve off the coast.

Experts are baffled as to why there were so many, and why they have ventured so close to the shore.

One theory is the young calves and accompanying adults have been attracted by the mackerel-rich waters.

From the reports we have received this school is one of the biggest to be seen in British waters Milford Haven Coast Guard

A so-called pod of about 40 dolphins was first spotted near Skomer Island - about a mile off the Pembrokeshire coast.

The island is owned by the Countryside Council for Wales and managed by the Wildlife Trust of South and West Wales, and is an internationally-important marine wildlife site, famous for its puffin and Manx shearwater populations as well as its dolphins and grey seals.

Day-trippers out sailing were delighted when they surrounded the cruise boat Dale Princess.

Cruise company director John Reynolds said: "It was amazing. Dolphins are one of the finest sights in the animal world and the trippers on the boat were given a ringside seat."

Juan Brown, the Wildlife Trust warden on the island, said the crew of a Dutch yacht had told him the dolphins followed their boat all the way from Ireland.

"It may be that they just got carried away, and ended up following the yacht all the way into the bay, and then decided to stay and have a play for a few hours," he said.

Kate Lock, Marine Conservation Officer for the Countryside Council for Wales said it was the first time they had been able to observe dolphins from such close range.

"The dolphins swam alongside the boat for about 10 minutes, slip streaming and bow riding," she said.

"We observed several metre-long juvenile calves accompanying the adults, which was wonderful to see.

"We have never had that experience before as the dolphins don't usually venture so close to Skomer."

Later on Wednesday a larger pod of over 100 dolphins was spotted further out to sea in St Brides Bay.

Mr Reynolds said since then the boat crew estimated around 300 are in the vicinity.

School

Common dolphins can regularly be seen around the offshore islands of Grassholm and the Smalls but it is not so common for such large numbers to be found so close to shore.

A spokesman for Milford Haven Coastguard said: "We have had lots of calls about the dolphin school.

"At this time of year dolphins can be spotted offshore - but not this close to the beaches and in such large numbers.

"From the reports we have received this school is one of the biggest to be seen in British waters."