Annual consumer price inflation plunged to a lower-than-expected 4.6 per cent in October from 6.7 per cent in September, official data showed on Wednesday.
The increase in consumer prices was the smallest in two years.
The Bank of England's forecasts and the consensus from a Reuters poll of economists had pointed to a reading of 4.8 per cent.
The ONS said the fall in the annual CPI rate was the biggest from one month to the next since April 1992.
Sterling fell slightly against the dollar after publication of the data, which showed key inflation measures watched closely by the BoE also falling by more than expected.
Core inflation, which strips out energy and food prices, fell to 5.7 per cent from 6.1 per cent, while service sector inflation also fell by more than the central bank had expected to 6.6 per cent from 6.9 per cent.
The data represented some rare welcome news for Sunak, who had promised to halve price growth this year before an expected 2024 election that opinion polls show his Conservative Party is likely to lose.
"In January, I made halving inflation this year my top priority. Today, we have delivered on that pledge," Sunak said on social media platform X.
Opposition MPs, and many economists, say the government blamed external factors for the rise in inflation, and then sought to take credit for the ensuing drop.
Despite the sharp fall in inflation last month, Britain retains the highest rate of consumer price growth among G7 nations - narrowly above France's 4.5 per cent - although Italy is due to publish its updated estimate for October later on Wednesday.
On Tuesday, US inflation data also came in weaker than expected, sparking a rally in government bond prices and sending other major global currencies higher against the US dollar, including the pound.
BoE Chief Economist Huw Pill said on Tuesday that the expected fall in inflation to just under five per cent would still leave it "much too high".
The BoE has stressed that it is nowhere near cutting interest rates from their 15-year high, even as the economy flat-lines close to a recession.
