The Staffordshire-based construction firm said the equipment market in Brazil, Russia, China and India was down 43% on the same period in 2014.
Staff across the UK could be affected and union consultation has begun, chief executive Graeme Macdonald said.
He said there was a "perfect storm" across worldwide markets which were either down or slowing.
"In the first six months of the year, the market in Russia has dropped by 70%, Brazil by 36% and China by 47%," the firm said.
"Parts of Europe are also struggling, with France down by 26%. Even the strong growth in the UK and North America has softened due to a fall in market confidence over the summer..."
Mr Macdonald said the business, which currently employs about 6,500 UK staff, has had to make some "difficult but necessary" decisions.
JCB currently has eight factories in Staffordshire, one in Derbyshire and two in Wrexham.
Gordon Richardson, of the GMB union, said: "These staff job losses are the first ripple from the downturn in world markets, including China, impacting on the UK economy.
"This huge wave of uncertainty is likely to lead to further job losses in the sectors that trade with the rest of the world.
"GMB will do everything we can to avoid compulsory redundancies and to mitigate the impact on the workforce."
JCB says it is to cut 400 jobs over the coming weeks after a "dramatic" slow down in world markets.