Premium bond holders will see their chances of winning £1m double with the introduction of a second jackpot prize, it was announced today.

From August this year two £1m prizes will be given away each month, as well as more than one million other prizes ranging from £50 to £100,000. The total prize fund will be worth £72.4m - £2m more than it is now.

National Savings & Investments (NS&I) said the introduction of a second £1m jackpot was the biggest change to premium bonds since the first £1m prize was introduced in April 1994.

It decided on the move following several years of significant growth in the amount of money invested in premium bonds.

People have invested more money in the Treasury-backed savings during the past six years than in the previous 42 years put together.

In April 1994, when the first £1m prize was introduced, a total of just £4bn was invested in premium bonds, compared with more than £26bn today.

As a consequence the odds of winning the jackpot had lengthened from one in four billion to 26bn to one. The second jackpot will reduce that to 13bn to one.

The number of prizes further down the scale has also been increased, so that an extra £1m a month will be paid out in £50 and £100 prizes.

The group said someone with average luck, who had the maximum £30,000 invested, could expect a tax-free return on their money of around 3.25% a year, the equivalent of 5.42% for a higher rate tax payer and 4.06% for a basic rate one.

"Our existing premium bond customers and those who are new to us are excited by the chance of becoming a millionaire, but also see it as a serious investment," said Alan Cook, NS&I chief executive.

"With premium bonds, you could win big money, but you definitely won't lose a single penny. It is this fact which, in an uncertain world, makes premium bonds so attractive to so many people.

"By offering two jackpots, NS&I is making premium bonds more attractive and thereby helping to encourage the savings habit in Britain."

Premium bonds were launched in 1957 and since then 123 million tax-free prizes worth £7.8bn have been given away.

Unclaimed prizes, many owned by people who have forgotten that they hold bonds or have moved house and not notified NS&I of a change of address, are now worth £23m. The bonds are held indefinitely by NS&I for winners to claim.

You can search to see if your bonds have won on NS&I's website