Ocado (OCDO) has announced a 15.4% rise in group revenues in the 12 weeks to 7 August to £314m.

Gross retail sales were up 13.6% in the period to £286.4m on an 18.9% rise in average weekly orders to 226,000. Average order size fell 3.4% to £107.94.

Tim Steiner, Ocado’s CEO, said: “Ocado has shown, once again, strong, steady growth as more customers across the country entrust us with their regular grocery shop.

“As the market remains very competitive, we are seeing sustained and continuing margin pressure and there is nothing to suggest that this will change in the short term. However Ocado’s combination of choice, competitive pricing, and industry-leading service has contributed to an increase in average orders by nearly 19%, our best volume performance in more than five years.

“We are confident that our commitment to further improving the customer experience through constant innovation, supported by our world-class proprietary technology, will allow us to continue to grow ahead of the online grocery market, and substantially ahead of the market overall.”

Ocado shares have plunged 10.2% to 289.3p despite the strong results on Steiner’s warning of continued margin pressure.