Amazon has reported lower-than-forecast second-quarter profits as it ploughed investment into its one-day delivery services.

The online retailer recorded second-quarter net income of $2.6bn, equating to $5.22 per share, which was above $5.07 this time the previous year but lower than analyst estimates of $5.57 per share.

The retailer’s net sales increased 20% to $63.4bn in the quarter to June 30, ahead of analyst expectations. However, Amazon flagged that profits would slip slightly in it current quarter.

The retailer’s operating income rose $3.1bn, ahead of $3bn the previous quarter.

The online retailer’s operating expenses increased 21% during the period as it ploughed Ā£800m into one-day delivery services, which chief financial officer Brian Olsavsky said has impacted earnings.

ā€œWe expect we will be working through that for a number of quarters,ā€ he said.

ā€œIt is a necessary strategy to compete with bricks-and-mortar’s speed advantage to the customer.ā€

The retailer posted its most successful Prime Day to date in the quarter, which spanned two days for the first time and said it had ā€œsurpassed the previous Black Friday and Cyber Monday combinedā€.

Amazon founder and chief executive Jeff Bezos said: ā€œCustomers are responding to Prime’s move to one-day delivery — we’ve received a lot of positive feedback and seen accelerating sales growth.

ā€œFree one-day delivery is now available to Prime members on more than 10 million items, and we’re just getting started.ā€