By David Milliken and Andy Bruce

LONDON (Reuters) - British retail sales rose at their fastest annual rate in more than 14 years in October as cold weather and Halloween boosted sales, but economists said Brexit effects were likely to weigh on spending next year.

Thursday's data reinforced the robust message so far from consumers since June's vote to leave the European Union, even if the one-off factors lifting demand were unlikely to last.

Retail sales volumes in October jumped by 7.4 percent, the biggest annual rise since April 2002, the Office for National Statistics said, outstripping all forecasts in a Reuters poll .

"Cooler temperatures boosted clothing sales as shoppers took their cue to purchase winter clothing, while the supermarkets benefited from Halloween," ONS statistician Kate Davies said.

Sterling gained modestly on the data. But the Bank of England and many economists fear higher prices caused by the Brexit hit to the value of the pound and slower jobs growth will eat into households' spare income.

"This rate of spending growth looks unsustainable, and there are a number of headwinds on the horizon," Capital Economics' Paul Hollingsworth said.

Thursday's figures showed store prices falling by the smallest amount since July 2014, and on Wednesday the ONS announced a slowdown in the rate of job creation.

It is unclear how quickly British consumers will feel the pinch. Finance minister Philip Hammond may cut taxes or boost public spending in his first budget statement on Nov. 23 to offset an expected fall in household incomes.

Sterling has fallen by more than 15 percent against the dollar since June's Brexit vote, pushing up import prices. But apart from a few high-profile exceptions, retailers have been reluctant to raise the price of everyday goods so far.

"Shops are waiting for each other to make the first move, unwilling to put off consumers ... This means the adjustment may be sudden when it happens," said Alasdair Cavalla, economist at consultancy CEBR.

Retail sales volumes jumped 1.9 percent on the month in October after edging up 0.1 percent in September, almost double the highest forecast in a Reuters poll of economists.

Retail sales are often volatile on a monthly basis. Clothing - which is particularly sensitive to the weather - gained 5.1 percent on the month, its biggest rise since March 2014, after mild weather dented demand in September.

But even looking at the three months to October as a whole, sales were 5.9 percent higher than a year earlier, the biggest rise since June 2002.

Tesco <TSCO.L>, Britain's biggest supermarket chain, reported on Tuesday its fastest annual sales growth for three years in the 12 weeks to Nov. 6.

However, Bank of England Governor Mark Carney warned this week that prices for British consumers will soon rise as stores start to pass on the effects of the fall in sterling since the referendum, eating into spending growth.

The central bank forecasts consumer price inflation will rise to 2.7 percent within a year from just 0.9 percent now, while real household consumption growth is predicted to slow to 1.25 percent in 2017 from 2.75 percent this year.

(Editing by Robin Pomeroy)