A new crack has opened in an Antarctic ice shelf where an iceberg a quarter of the size of Wales is already poised to break off, scientists have said.
Last year, a rift in the Larsen C ice shelf grew suddenly by around 18km (11m), leaving the iceberg "hanging by a thread".
The rift has continued to grow this year and is currently 180km (110m) long.
Now, satellite images have shown that a second branch of the rift has opened up.
The new crack is 15km (9m) long, according to researchers from the Swansea University-led Midas project, which has been studying the stability of the ice shelf for three years.
"While the previous rift tip has not advanced, a new branch of the rift has been initiated," said Professor Adrian Luckman of Swansea University College of Science, who heads the monitoring group.
"This is approximately 10km (six miles) behind the previous tip, heading towards the ice-front."
The Larsen C ice shelf is the most northern major shelf in the Antarctic Peninsula.
The iceberg set to break off has an area of 5,000 sq km (1,930 sq m) and will be 350m thick - making the split one of the biggest of its kind.
The new crack marks the first significant change to the rift since February, Professor Luckman said.
But he added: "Although the rift length has been static for several months, it has been steadily widening, at rates in excess of a metre per day."
About 20km (12m) of ice connects the iceberg to the rest of the ice shelf, the researchers have said.
"When it calves, the Larsen C ice shelf will lose more than 10% of its area to leave the ice front at its most retreated position ever recorded," said Professor Luckman.
"This event will fundamentally change the landscape of the Antarctic Peninsula."
Environmental experts have expressed worry that further shedding of ice shelves around the frozen continent due to rising temperatures will lead to raising world sea levels.