The ONS said the recent spate of energy bill increases would likely lead to similar increases in inflation as seen last year, when utility price increases added 0.45% to CPI.
Rising fuel prices also put upward pressure on CPI last month, when petrol rose by 3.9p a litre between August and September compared with a fall of 0.3p a year ago, according to the ONS.
Experts fear rising food prices will also push inflation back up.
The latest figures showed the Retail Prices Index (RPI), which includes housing costs, also eased back last month, to 2.6% from 2.9% in August.
Last month's inflation fall will come as bad news for basic state pensioners and those on benefits, as the Government uses September's CPI figure to calculate their payment increases the following April.