Experts examining the 33ft (10m) long animal, which died about 875 yards (800m) from the shoreline, say they are 95% sure it is a female sei whale.

The animal was trapped in shallow water near the East Yorkshire village of Skeffling, on the north bank of the River Humber.

Andy Gibson, of the Yorkshire Wildlife Trust, said Sei whale strandings were very rare.

He said there had only been three strandings of this species in UK waters in the last 20 years.

''It is sad. It was in shallow water of about 1.2m (4ft) to 1.6m (5.25ft), making contact with the bottom,'' Mr Gibson said.

''This was about 800m offshore.

''When it gets in that situation it rolls onto its side and it can cover its blow hole.''

But Mr Gibson said: ''It is sad but we will be able to learn a lot from it.''

The whale is the latest of a number to have died in the Humber estuary area recently.

Earlier this month, a young fin whale - a relative of the sei - was stranded at Immingham, North East Lincolnshire, and subsequently washed up dead near Spurn Point.

And another dead whale was spotted in the river mouth in the last few days but has yet to wash ashore.

The Yorkshire Wildlife Trust has noted a rise in whale sightings generally in 2011 but no-one is sure why there may be an increase in the mammals in the North Sea.

Over the summer, a pod of up to 10 Minke whales were spotted regularly off the North Yorkshire coast between Whitby and Scarborough.

Whale experts admit they do not know why there has been an upsurge in sightings and strandings.

Some believe it could be due to changes in ocean currents bringing colder streams of Arctic water into the shallow North Sea.