Robert Wiseman Dairies has posted an increase in sales and post-tax profits but has maintained its profit warning.

Turnover rose 6.8% to £452.8m in the six months to 2 October 2010 as it announced its interims on Monday. Although pre-tax profit fell 3.5% to £20.2m it was mitigated by a lower tax charge, resulting in an overall 1.5% rise in profit after tax to £15.1m. Operating margin fell to 4.6% compared with an adjusted operating margin of 5.1% in the same period last year.

The company maintained its forecast of a £7m hit to trading profit in its next set of results. "While sales volumes had been maintained, margins across all sectors had been eroded," it said, adding it would address reduction in margins through ongoing cost reductions and efficiencies.

Wiseman also announced it had picked up a contract to supply an additional 35 million litres of milk per annum to Tesco.