Falling store prices and Rugby World Cup promotions helped boost retail sales in September at the fastest monthly rate since December 2013, according to official figures.

The Office for National Statistics (ONS) said the quantity bought in the retail industry is estimated to have increased by 1.9% compared with August.

Year-on-year sales registered a 6.5% increase.

ONS head of retail sales statistics Kate Davies said: "Falling in-store prices and promotions around the Rugby World Cup are likely to be the main factors why the quantity bought in the retail sector increased in September at the fastest monthly rate seen since December 2013.

"The retail sector is continuing to grow with September seeing the 29th consecutive month of year-on-year increases."

Ian Gilmartin, head of retail and wholesale at Barclays, said: "September's ONS retail statistics show that it was a really impressive month for retailers, accelerating August's strong numbers.

"Hosting the Rugby World Cup provided a timely boost to sales and helped the industry set a solid platform for the all-important Christmas period.

"Retailers are now readying themselves for Black Friday and finalising their preparations for Christmas, and it's clear from my conversations with clients that there is a growing optimism in the industry, with retailers increasingly hopeful that higher levels of disposable income resulting from recent drops in food and fuel prices will lead to a positive outcome in the season ahead."

The ONS said average store prices, including petrol stations, were down 3.6% on September last year, the 15th consecutive month of year-on-year falls.

The amount spent in the retail industry increased by 2.7% compared with September last year, and by 1.4% compared with August.

The value of online sales increased by 15.2% on a year ago and by 4.5% compared with August.

Ben Perkins, head of consumer business research at Deloitte, said: "Today's figures further support our view that consumer confidence is growing.

"September's increase in the value of online sales is poignant, particularly as we are now just over a month away from what is likely to be another record-breaking Black Friday."

Chris Towner, chief economist at HiFX, said: "This time of year is famous for beer festivals and it looks like the biggest festival this year has been in the UK in celebration of rugby.

"With falling prices and wages up 3% from a year ago, the British population certainly has reason to cheer and this recent surge in the retail sector is expected to add 0.1% on to GDP in the third quarter."