The 12-month average growth is 11.9 per cent. In May 2015, Online sales represented 17.4 per cent of total non-food sales, against 16.2 per cent in May 2014.
British Retail Consortium director general Helen Dickinson said: "Over the last 12 months online has contributed more to non-food sales growth than bricks and mortar stores, rewarding the substantial investment retailers have piled into new channels in recent years."
KPMG retail head David McCorquodale said: "As investment being made by many retailers in omnichannel systems remains unabated, this reflects the continued importance of this channel for consumer and retailer alike."
UK retail sales were flat on a like-for-like basis from May 2014, when they had increased 0.5 per cent on the preceding year. On a total basis, sales were up 1.1 per cent, against a 2.0 per cent rise in May 2014 and behind the 3-month and 12-month averages of 1.8 per cent and 1.4 per cent respectively.
Dickinson added: "May witnessed a slow-down in sales growth, below the three-month and twelve-month average. This was mainly due to fashion sales, which experienced a decline compared with the same month last year, where we saw record demand.
McCorquodale said: "With Easter distortions behind us, May’s retail sales figures were disappointingly flat.
"Buoyed by a conclusive result in May’s General Election, the housing market picked up leading to stand out performances for furniture and homeware sales. As economists predict another housing boom for the second half of 2015, these trends could be set to continue.
"Many retailers stretched out summer sales events and deepened discounts in order to try and entice consumers through the door.
"Looking ahead, June may be a tough month of comparables for some with the start of the 2014 World Cup in June last year boosting sales in certain areas. However, with consumer confidence nearing pre-crisis positivity, retailers will be hoping that the improving job market, low inflation rates along with a dollop of summer sunshine will provide a welcome boost."
HTA horticulture head Raoul Curtis Machin said May had been "reasonable" with "no-one reporting shocking figures" but sales had been slower in the north because of the cold weather.
GCA chief executive Iain Wylie said: "If centres are up, its only marginal. It's been a mixed bag. Bank holidays have been reasonable but no records are being broken and there are no double digit increases."
He added: "Plant sales not reached the record heights of 2009/10 but have been relatively solid this year across the board. May dampened spirits and meant for a slow start to the bedding season."