Most UK small businesses are failing to reap the rewards of a huge boom in mobile shopping because their websites are not optimised for use on smartphones and tablets, new research has shown.

Shoppers are predicted to double the amount they spent on purchases made on mobile devices to £27bn this year. That figure is predicted to hit £43bn by 2020 but only 18 per cent of the UK’s small businesses have mobile-friendly websites, a survey by PayPal has found.

The research reveals a growing gap between what shoppers want and expect from mobile shopping and what small businesses are actually offering. As mobile becomes the key growth area for online spending, small businesses risk being left behind larger rivals unless they make changes, PayPal said.

Some businesses are unaware of the importance of mobile sales, with 33 per cent of respondents saying they don’t need a mobile website because they “do well enough as it is”. However, consumers said the lack of a mobile-friendly website was their number one frustration when buying something on their smartphone.

UK businesses are likely to become increasingly reliant on mobile sales, with almost one third of Brits saying they expect to use their smartphones to shop more often in the next 12 months.

Firms particularly risk losing the younger generation of consumers with 44 per cent of 16 to 25-year olds saying they plan to buy more on their mobile. Six in 10 millennials said they look for potential purchases on their mobile device every day.

One example is to implement online payment systems instead of relying on traditional methods such as bank transfer, which 70 per cent of small firms do, according to the research. More than half of companies rely on cash and 45 per cent say cheque payments are still important.

Implementing a payment system makes buying goods far simpler and gives the reassurance of security and a recognised brand, Ms Longfield said. This in turn encourages shoppers to spend more. For over a third of businesses, the average value of an individual sale is between £10 and £30, but the research shows that consumers are willing to pay up to three times that on digital devices: £84 on a smartphone, £103 on a tablet and £147 on a laptop.