A marine heatwave could reach "extreme" levels around parts of the UK later this week, according to the Met Office, raising concerns for marine life.

Long periods of sea heat can trigger mass die-offs among some seagrasses, shellfish and other species, as well as encouraging greater numbers of warm-water creatures including octopus.

The heatwave is currently strongest off the coasts of eastern and southern England, and sea temperatures could reach 4-5C above average in places.

The marine heatwave has been fuelled by the "heat domes" that brought record-breaking air temperatures in May and June, on top of long-term ocean warming due to climate change.

Important marine habitats such as seagrasses and kelp forests are suited to cooler waters and can experience high levels of heat stress when temperatures surge.

This can reduce their growth or even lead to mass mortality events, which could have knock-on effects for species that rely on these habitats.

The UK has experienced marine heatwaves - prolonged periods of unusually high sea surface temperatures - before.

But scientists fear this event could be particularly intense and long-lasting, particularly for parts of the English Channel and the southern North Sea.

"We're starting to see temperatures now that we would expect to see at the height of summer, [at the] end of August," said Dr Zoe Jacobs of the National Oceanography Centre in Southampton.

"So, if this event keeps coming… we might start seeing some serious impacts on ecological systems."

Researchers have also noticed longer-term shifts in the UK's marine life. Cool-adapted species such as cod are generally moving further north, while numbers of some warm-water species such as octopus have risen around south-west England.

While natural cycles and fishing practices can affect the abundance of these creatures, many scientists point to the crucial role of warming seas.