Tilney believes the figures, which show 697,000 fewer ISA accounts subscribed to in 2017/18, were evidence that demand “continues to crumble.”

But AJ Bell pointed to different numbers in the research which highlighted that the money saved in ISAs had actually increased to £69.3bn.

Jason Hollands, managing director of Tilney, said: “The data released today updates the ISA statistics to include the 2017/18 tax year.

“By way of reminder this was a year when UK interest rates were at an all-time low, with base rates rising from 0.25% to 0.5% in November 2017, while global stock markets were buoyant against a background where politics loomed large in the minds of many investors.

“The latest data shows that 10.8 million ISA accounts were subscribed to 2017/18, down from 11.1 million in the previous tax year – a 10% decline.

“But look behind this, it was Cash ISAs which took the hit in demand, with 697,000 less subscribed to, while stocks & shares ISA rose in popularity, with 246,000 more accounts opened than in the previous tax year.

“Junior ISAs, the scheme to help build up savings pots for children, grew in popularity too, with £902 million subscribed into 907,000 accounts subscribed to, up from 794,000 in the 2016/17 tax year.

“The declining interest in Cash ISAs is understandable given the abysmally low interest rates on offer at a time when inflation spiked, put household finances under pressure and meaning real returns on cash savings were negative.

“But another reason for the demise in enthusiasm for Cash ISAs is that for most Britons these have now become, frankly, surplus to requirements as basic rate taxpayers no longer pay tax on their first £1,000 of savings interest anyway and higher rate taxpayers can earn up to £500 of interest tax free.

“With such a framework in place, there is little need to rush out and eagerly open a cash ISA account.”

Laura Suter, personal finance analyst at investment platform AJ Bell, said: “The number of people saving into an ISA has fallen for another year, taking the total number of ISA accounts to its lowest level since the 1999-00 tax year.

“This comes after a similar drop the previous year, with an almost 13% fall in the number of ISAs opened.
 
“However, the amount of money saved into ISA accounts in 2017-18 has actually increased in the past year, to £69.3bn – so fewer people are using the accounts but those that do are saving more.

“This is a surprise turnaround following a 23% fall in the amount of money saved within an ISA between 2016-16 and 2016-17 – although remains below 2015-16’s figures.
 
“The majority (72%) remains in cash ISAs.

“At the paltry interest rates being offered by many banks and building societies, coupled with currently high inflation, the bulk of this money is likely losing spending power in real terms.”
 
Clare Francis, director of savings and investments at Barclays Smart Investor believes the future is bright for ISAs.

She said: “The record amount invested last year proves that investment ISAs are now a truly attractive option for people looking to maximise their long-term savings.

“Total investment into stocks and shares ISAs rose by nearly £6.4bn to £28.7bn last year.
 
“Low interest rates have of course played their part in spurring people to look for ways of diversifying their savings, but the growing popularity of investment ISAs also reflects a shift to a more long-term view among savers.

“People are saving towards their life stage goals, like buying a house or planning for their retirement.”
 
She added: “Cash ISAs still play an important role in any tax-efficient savings plan, but the accessibility of investment ISAs gives people more choice and control over how they make the most of their money.”