Shares in womenswear retailer Bonmarché plunged by a quarter after the firm issued a profit warning as “unseasonably hot weather” in September melted sales of its autumn ranges.

The Grange Moor-based retailer said it expects like-for-like sales at its shops to fall by 8% in the second quarter and first half of the year. As a result, Bonmarché said full year pre-tax profit will come in at between £5m and £7m.

The firm said: “Trading in September has been extremely poor, largely as a consequence of the unseasonably hot weather which has not favoured sales of our new autumn ranges.

“We approach the beginning of the second half of the year facing considerable uncertainty as to market conditions. The hot September has prevented us from gaining a representative measure of the strength of the autumn ranges and our perception is that the clothing market generally has become more challenging.”

The statement said: “Notwithstanding difficult trading conditions in July and August, overall performance was in line with management’s expectations for the full year. However, trading in September has been extremely poor, largely as a consequence of the unseasonably hot weather which has not favoured sales of our new autumn ranges.

“The hot weather conditions have resulted in the strong sales of residual summer stock, however, and this will result in an end of season summer stock holding which is below last year’s level, despite the generally poor summer season.

In June, the value chain said full-year pre-tax profits dropped by more than a fifth to £9.6m, down from £12.4m in 2015, as it grappled with an “unusually difficult year”.

New boss Helen Connolly, formerly senior buying director for Asda’s George clothing business, who took up the role at Bonmarché earlier this year, said she was formulating plans for the future, adding: “The direction of travel is right, but the effectiveness of execution needs to improve.

“My plans are therefore likely to focus on improving the clarity of the customer proposition and operational improvements in all channels rather than a major strategic repositioning.”

She said she was encouraged by the enthusiasm and commitment of the Bonmarché colleagues and was confident that despite current difficult conditions, the business would resume growing in the full year 2018.

She added: “I will provide a more comprehensive update in relation to my strategy in due course.”

Kate Calvert, analyst at Investec, said the shares will “remain friendless” until Ms Connolly outlines her strategy and performance improves.

Bonmarché employs more than 3,000 staff across almost 300 shops and concessions, including one at New Street in Huddersfield.