The quarter of a million pound question - have you used your ISA?

A couple who have both invested their full ISA allowance since the tax-efficient savings vehicles were launched in 1999 could have a nest egg of more than a quarter of a million pounds, according to research from Fair Investment Company.

"If an individual had invested their full allowance into a stocks and shares ISA each year since 1999 - £7,000 until 2007, £7,200 from 2007 until 2009 and £10,200 in 2010 - they would have invested a total of £87,600," explains Julie Smith, savings analyst at Fair Investment Company.

"Assuming growth of 7.00% pa(net of charges and not taking account of inflation during this period) , the gain on this investment would be £50,282, creating a total pot of £137,882. If their partner had done the same, the result is more than a quarter or a million pounds sheltered from the tax man."

Julie says that ISAs are often not considered seriously by investors because the gains seem small, but says the cumulative gain is worth it.

"When you look at the tax advantages on an ISA for a single tax year, they may not seem worth it, but when you look at how much you could gain if you invest into an ISA year after year, it suddenly becomes very appealing."

Fair Investment Company's research shows that a husband and wife who have both invested their full allowance into an ISA since their launch in 1999 would have invested £87,600 each by now, which is a joint pot of £175,200 sheltered from the tax man. And that is before any growth is taken into account.

Assuming growth of 7.00% pa (net of charges and not taking account of inflation during this period), this pot would have accumulated a gain of more than £100,000, creating a joint nest egg of £275,764.

"It is this sort of illustration that shows why many people are turning to ISAs as an alternative way of funding their retirement. Because as you can see, compound interest really adds up - £275,764 is a lot of money, and could have been accumulated by a couple who have both simply invested their full ISA allowance every year."

With the ISA deadline coming up (5 April), Julie is urging those who haven't yet made the most of this year's ISA allowance to do so before it's too late.

"It's not realistic to assume that everyone can afford to invest the full £10,200, but investing in an ISA, even if it's just a small amount, should really be your first option because of the tax-efficient status it provides."