Details of a new savings product which will fund the government's environmental projects have been revealed - except for one key factor.
Anyone aged 16 or over will be able to put between £100 and £100,000 into Green Savings Bonds from National Savings and Investments (NS&I).
Money will be locked in for three years but, significantly, the interest rate has yet to be announced.
That, and when the bonds go on sale, will be unveiled later in the year.
Anna Bowes, of comparison site Savings Champion, said: "It is frustrating that we were told this would be available in the summer but that date is being stretched."
She said the interest rate was unlikely to be very competitive because the bonds would be attractive to people who wanted to save safely in green projects.
What is the Green Savings Bond?
Chancellor Rishi Sunak announced in the Budget that the new savings product would go on sale in the summer.
It is designed to raise billions of pounds for environmental projects such as wind and hydrogen power, as part of plans to hit the government's net-zero carbon emissions target by 2050.
The chancellor also hopes it will assist with green shoots of economic recovery after the pandemic by creating jobs in this sector.
In effect, the money will go to the government to spend on these products, rather than the public investing directly in a specific environmental scheme.
The new Bond will go on sale via NS&I - the government-backed provider of Premium Bonds. It has been heavily criticised by savers and MPs for its customer service performance during the pandemic.
However, it is attractive to savers as their money is 100% guaranteed to be safe by the government. Up to £85,000 per person, and per bank, building society or credit union is guaranteed for savers elsewhere under the Financial Services Compensation Scheme.
What do savers get?
The terms - published on the NS&I website - state that savers aged 16 and over can apply online and each put in between £100 and £100,000. Anyone unable to operate online can phone NS&I for help.
To receive interest, they must not withdraw their savings for three years.
In recent years, savers have been starved of any kind of decent returns owing to cheap rates of borrowing.
Significantly, the Green Savings Bond interest rate has not yet been declared.
The average rate for three-year bonds on the market is 0.76% a year. The most generous interest rate for a three-year bond at the moment is 1.26% a year.
Is this the only product of its kind?
The Treasury has described it as a world-first. Savings Bonds are certainly nothing new. The Investment Guaranteed Growth Bond (IGGB) was sold via NS&I for a year from April 2017. That was a three-year bond, with an interest rate of 2.2%.
There are a number of "green" savings products available to savers, some of which have market-leading interest rates.
They include the Gatehouse Bank Green Saver and Oxbury Bank Forest Saver, which fund tree planting projects. Triodos Bank and the Ecology Building Society also use money raised from savers to lend to environmental projects.
Other specific green investment prospectuses are also marketed in the UK, although they often rely on the projects to be successful before paying any kind of return.