In an episode of the BBC series, the young animal was shown lying dead after its mother had mistaken the plastic toothpick for food.

The television presenter spoke of the threats Earth is facing, including the eight million tons of plastic dumped into the sea each year, global warming and the rate of overfishing, in a column in the Radio Times.

There are concerns that more than a million birds and 100,000 sea mammals and turtles die every year from eating and getting tangled in plastic waste.

Sir David, 91, also echoed a previous call that he hoped US President Donald Trump would reconsider his threat to withdraw from the Paris Agreement.

He wrote that "never before have we been so aware of what we are doing to our planet – and never before have we had such power to do something about it".

"Surely we have a responsibility to care for the planet on which we live? The future of humanity, and indeed of all life on Earth, now depends on us doing so," he added.

"Plastic is now found everywhere in the ocean, from its surface to its greatest depths," Sir David wrote.

"There are fragments of nets so big they entangle the heads of fish, birds and turtles, and slowly strangle them.

"Other pieces of plastic are so small that they are mistaken for food and eaten, accumulating in fishes' stomachs, leaving them undernourished."

The broadcaster added: "Our wellbeing is inextricably bound up with the health of the oceans."

He said "all is not yet lost" and told people to "reduce the amount of plastic that we use in our everyday lives".

Sir David praised countries for coming together in 2015 to sign the Paris Agreement, but added: "It is true that since then the United States has threatened to withdraw from that decision.

"Let us hope that Trump will eventually recognise that the Paris Agreement was not about Pittsburgh, or even Paris, but the entire planet," he said.

Radio Times is out now.