Ocado, the online grocer, has warned of a smaller-than-expected rise in full-year profit margins – after it invested in improving customer service and saw sales growth slow in its third quarter.

The company said in an interim management statement this morning that gross sales had risen by 16.9% to £148m in the 12 weeks to 7 August, down from 17.2% growth in the second quarter.

Ocado said it received 110,945 orders per week in the period, with customers spending £111.08 on average.

Tim Steiner, chief executive of Ocado, said: “In spite of the tough economic environment, our sales are growing substantially and we remain focused on improving the range, value and service for our customers.

“We are continuing to expand the capacity of our first fulfilment centre in Hatfield, with significant progress made over the summer months.”