Anyone with a cash ISA issued warning over making mistake that'll cost them £620 This is Money has reported how its personal finance experts "aren’t big fans of this short-term bonus bonanza." This is Money has reported how its personal finance experts "aren’t big fans of this short-term bonus bonanza." Anyone with a cash ISA has been warned over s avings bonuses which "have sparked madness". This is Money has reported how its personal finance experts "aren’t big fans of this short-term bonus bonanza." They say: "We’ve questioned why providers are allowed to list these deals with a high headline AER (annual equivalent rate) when this includes a three-month bonus and so they are guaranteed not to pay that for a whole year." ‌ It warned Trading 212 is currently the market-leading cash ISA and pays 5.05 per cent including a 0.7 per cent bonus, while until Wednesday Plum paid 5.68 per cent with a 2.14 per cent bonus. Article continues below READ MORE Exact dates next UK mini-heatwave start and end with 72-hour scorcher set to hit But the Plum bonus rate has now dropped to 5.04 per cent, including a 1.5 per cent bonus. This is Money advised: "Of course, these high cash Isa rates are also teasers to get savers to sign up and then use the apps to invest, so that’s something you should be aware of too. ‌ "But some of the apps also offer very cheap Isa investing, so that’s not necessarily a bad thing." Tembo claims, that rather than saving into an ISA offering an introductory bonus rate, UK savers could increase their annual interest returns by nearly three times by transferring their cash ISAs to a provider offering a better underlying interest rate. A cash ISA balance of £20,000 would equate to an additional £620 interest being paid each year. Richard Dana, founder and CEO, Tembo said: “Boosting the headline interest rate in the short term in order to attract customers is confusing and unfair. "These marketing gimmicks are not widely understood by savers who can lose out on better interest rates if they remain loyal to their savings provider.” Article continues below “Bonus or teaser rates are used by companies to attract new customers, in the knowledge that the majority of those customers will not be motivated to move their money in six-12 months time. "They also clearly penalise loyal, longstanding customers who end up earning less interest than new customers." Richard said: "It is vital that customers are aware of the impact of these introductory offers so they can make an informed choice."