LONDON/NEW YORK (REUTERS) - A London-based start-up has joined up with Mastercard to launch a payment card that allows users to retroactively choose a different credit or debit card for a purchase they have already made, in what they called "financial time travel".

Starting on Tuesday (July 4), Curve will offer customers the chance to "go back in time" and switch the card used for any given transaction for up to two weeks after the purchase, in what it said would be a world first.

The start-up also said on Monday that it was planning to announce backing from two large international banks soon, but declined to provide more detailed information.

Curve, which has a patent pending for the new tool, provides customers one card that can aggregate all of their existing Mastercard and Visa Inc payment cards. Users can carry only the Curve card and switch between the cards they want to use for a purchase through a mobile phone app. Founded in 2015, the start-up is among a growing group of young companies that seek to make better use of digital technologies to make payments and banking products more user-friendly.

The start-up's chief executive Shachar Bialick said the new "time travel" functionality will give customers more time to chose the most convenient form of payment for a given transaction.

For example, users might have funds available in their current account days after a purchase paid for with a credit card - and would therefore prefer to switch the transaction to a debit card to avoid interest fees.

"The user can manage their cash flow much better," Bialick said. "It's very hard to make the decision at the point of sale." The functionality would also enable users to take better advantage of card rewards, he said.

While users would be able to switch cards, the change would happen within Curve's platform, so merchants would only see the original transaction and would not be charged card processing fees twice.

Rewards gained through the first purchase will be reversed if that card is changed, Curve said.

Curve, which is still in "beta" testing mode, has already had more than 50,000 sign-ups, with the cards having been used for over £50 million (S$89.4 million) in transactions so far. It plans to fully launch this year.