Two major online retailers have been accused of misleading consumers in the sale of products online.

Amazon and Shein were identified by Which? as using prompts online which are recognised as pressure tactics by behavioural scientists. They can include countdown deals, low stock alerts and popularity alerts.

In tests by the consumer watchdog, claims made in the prompts did not always reflect its own results.

When putting lightning deals to the test on Amazon Marketplace, Which? looked at a balance board advertised for £134.99 (RRP £209.99), with the deal supposed to end in six hours on 3 August. The next day, it was still £134.99 – and every day for the five days they checked.

Countdown clock deals also repeatedly reset on Shein, where an eyeshadow stick was on a flash sale with 49 per cent off (£2.26, reduced from £4.49). The sale was meant to end at 5pm on 4 August. But it was extended until 7 August, and then again to 8 August. All the while, the price remained at £2.26.

The findings come after the Competition and Markets Authority (CMA) launched a string of investigations over concerns that their use of countdown clocks could mislead shoppers.

Hayley Fletcher, the CMA's director for consumer protection enforcement, told Which?: “The law requires that these claims are genuine and accurate, and authorities like the CMA can take tough enforcement action when that's not the case. The rules are really clear if you're making a claim, then that needs to be true. The law is clear, and traders must abide by it.”

The tactics are rooted in behavioural science, according to Jeff Bray, a professor of consumer behaviour at Bournemouth University.

He told Which?: “There are studies that show increased brain activity around the notion something might sell out. Missing out is far more powerful as a motivator than gaining. We should always be sceptical about marketing claims that can’t be verified. And if they are really selling out so quickly, why do they need to shout about it?”

Lisa Webb, Which? consumer law expert, said: “Countdown clocks, low-stock alerts and claims that certain items are extremely popular are designed to make people act fast, but our research found shoppers often do not trust them, and with good reason.

“At a time when household budgets are under strain, consumers must have confidence that businesses are playing by the rules and that they won't be rushed into a purchase they later regret.”

has approached Shein for comment. The retailer told Which?: "Like every retailer, offline and online, we seek to offer an enjoyable and rewarding shopping experience, including through regular sales and promotional events, while providing clear and useful information so that consumers can make informed and discerning shopping choices.

“We have clear internal policies in place to ensure that our promotional practices are transparent and comply with applicable laws and regulations, including guidelines governing the use and duration of flash sales.

“As soon as we became aware of this incident, we launched an internal review and promptly fixed the error. We have taken action to strengthen our internal training and controls to ensure that our promotions are implemented consistently and in line with CMA guidance.”

has contacted Amazon for comment. The retailer told Which?: “Amazon works hard to offer customers low prices and genuine deals, and to provide clear and accurate pricing information so customers can make informed purchasing decisions.

“Deal timers reflect when a specific deal is scheduled to end. In the examples raised, the deals ended as scheduled. Third-party sellers set their own prices on Amazon. The prices that followed the deals were independent seller pricing decisions outside of the deals programme.”