A nuclear war between India and Pakistan could kill up to 125 million people and then tip the world into a decade of starvation as smoke blocks sunlight, researchers have estimated.

If the neighbours attacked each other with a significant proportion of their growing nuclear arsenals, millions would die instantly from the blast and then firestorms would rage through cities. The fires would send huge quantities of smoke into the atmosphere, devastating agriculture as it blocked sunlight and cut temperatures.

Escalating tension between the countries because of their rivalry over Kashmir has this year underlined the threat the threat of war between the nuclear-armed states.

They have fought two wars over the disputed Himalayan territory and fought air clashes in February.

“India and Pakistan are of special concern because of a long history of military clashes including serious recent ones, lack of progress in resolving territorial issues, densely populated urban areas, and ongoing rapid expansion of their respective nuclear arsenals,” according to a paper in the journal Science Advances.