The confirmation comes after two months of rumours that the dongle and app-based mobile card reader start-up would secure the investment valuing it at up to $3.25 billion.

Starbucks confirmed its $25 million commitment a few weeks ago as part of a wider deal that sees the coffee chain commit to using Square's mobile payments technology in its stores and entrusting it with all US card transaction processing.

Jack Dorsey's firm says that it has seen huge growth over the last 12 months, with its workforce growing from 150 to 400 and annualised payments processed up from around $1 billion to over $8 billion. The company's next stage will now see international expansion later this year.