Apple is expecting its Christmas revenues to be several billion dollars lower than expected.

The tech giant's CEO Tim Cook says the lower revenue estimates are partly a result of America's recent trade war with China.

That, combined with a slowdown in the Chinese economy, has led to the company selling far fewer iPhones there than anticipated.

In a letter to investors, Mr Cook said Apple "did not foresee the magnitude" of the economic challenges in major emerging markets such as China.

He said: "As the climate of mounting uncertainty weighed on financial markets, the effects appeared to reach consumers as well, with traffic to our retail stores and our channel partners in China declining as the quarter progressed. And market data has shown that the contraction in Greater China’s smartphone market has been particularly sharp.

"While Greater China and other emerging markets accounted for the vast majority of the year-over-year iPhone revenue decline, in some developed markets, iPhone upgrades also were not as strong as we thought they would be."

He stressed there are still positive results for the company.

He highlighted there are now more Apple devices being used than ever before, and noting that growth from the company's non-iPhone business - including from its Apple Watch and AirPods 'wearable' products - has grown by 19%.

However, an unexpected revenue warning from Apple - which last year became the first company to achieve a $1 trillion valuation - is likely to fuel further stock market uncertainty following a turbulent December.