27 million mobile phones and 12 million tablets will be sold in the UK this year - some five million devices fewer than 2013, predicts CCS Insight.
The analyst also estimates mobile phone sales will drop 11 per cent this year, while tablet sales will fall 14 per cent. However, its infographic shows that sales are expected to rise again in 2015 and beyond, with CCS predicting the number of tablets in use to double by 2017.
Sales of mobile phones dropped 20 per cent in the first months of 2014 compared with the same period in 2013. Meanwhile, tablet sales, which broke all records in previous quarters, slumped 17 per cent.
CCS say this is the first time such a sharp decline has been recorded in the UK outside a recession.
Marina Koytcheva, director of forecasting at CCS Insight, said: "The slowdown in tablet sales should come as no surprise to the industry. We warned back in December that the spectacular tablet growth we've been seeing in the last two years was unsustainable. With replacement rates for tablets slower than those of smartphones, a temporary lull was inevitable."
The forecast revealed that replacement sales of mobile phones are slowing, with consumers having little reason to upgrade as regularly as they have in the past.
CCS Insight expects competition to reach 'unprecedented' levels in the second half of 2014. It reckons consumers will look to replace old PCs and adopt wearable tech, and that the market will get a lift when Apple unveils the next iPhone.
Ben Wood, Chief of Research at CCS Insight, added: "This is a double whammy for device manufacturers. No one anticipated we'd see so much pressure on both tablet and smartphone sales at the same time. In response to slowing sales, consumer electronics companies will be tempted to step up their marketing efforts and spoil buyers with a wealth of attractive promotions and bundles. Early signs of this have already emerged, with cheap tablets or wearables being thrown in with a smartphone sale to sweeten the deal".
CCS Insight CEO Shaun Collins said: "The Brits love their gadgets and now they have to spread their money across more devices. It's no longer just about having the latest and greatest smartphone.”