The latest British Retail Consortium/KPMG retail sales monitor showed a negative trend heading into the key Christmas trading period.
Meanwhile, separate figures from Barclaycard revealed that consumer confidence also dipped in the month amid concerns over fuel and supply shortages.
The BRC/KPMG report showed that total sales increased by 0.6 per cent in September against the same month last year, compared with an average of 3.1 per cent growth for the past three months.
Like-for-like sales were 0.6 per cent lower for September compared with the same month in 2020.
BRC chief executive Helen Dickinson said: “September saw the slowest retail sales growth since January, when the UK was in lockdown.
“There are signs that consumer confidence is being hit as the fuel shortages, combined with wetter weather, had an impact in the second half of the month.”
She added that this particularly affected larger purchases, such as furniture and homeware.
Online non-food sales were lower for the month as more people returned to high street stores.
Over the three months to September, non-food retailers, such as fashion and homeware stores, saw total sales rise by 3.8 per cent.
Meanwhile, food stores saw sales rose by 2.3 per cent for the quarter.
Elsewhere, Barclaycard said its latest consumer spending data for September showed that consumers are starting to feel the impact of rising prices on their finances.
It revealed that nearly half of shoppers said they saw empty shelves in supermarkets, while 18 per cent found it harder than usual to find fresh fruit and vegetables.