William Morrison Supermakets, Britain's fourth-largest grocery store chain, reported disappointing Christmas sales figures as investors prepare for a critical week of updates from major food retailers.

The Bradford, England-based group said overall like-for-like sales for the 6 weeks ending on 30 December declined by 2.5 percent. Sales not including fuel fell by 0.9 percent from the same period last year, the company said in a trading update.

"Our 130,000 colleagues have done an outstanding job serving our customers great value food this Christmas and I would like to thank them for their dedication and hard work," said Chief executive officer Dalton Philips in the statement. "In a difficult market our sales performance was lower than anticipated, but we have a strong business and significant opportunities to advance our strategy, as we accelerate our multi-channel offer".

Morrison shares, however, rose in early London trading after the company maintained its full-year profit outlook of around £913m even as it described the 2012 consumer and sales environment as "challenging". Full year earnings are expected to be announced on 14 March.

The stock jumped 0.5 percent to change hands at 258.2 pence each following the company's trading update. The shares have declined around 14.5 percent in the past year.

The group said last week it had reached an agreement with two British televion presenters - known as Ant & Dec - to front a sales campaign that would showcase the company's product offering in a more traditional light. It had also asked the government for an extension in opening hours on 23 December - a Sunday, which trading is only permitted for a maxium of six hours - in order to help boost last-minute sales.

The figures kick-off a busy week for investors in UK retail shares - and those seeking a baromoter of broader retail sales for the month of December. Tesco plc, Britain's largest supermarket chain, will report its Christmas sales figures on Thursday 10 January - the same day as clothing retailer Marks & Spencer - while rival J Sainsbury Plc will update on Wednesday 9 January. Department store chain Debenhams will post sales figures on Tuesday 8 January.

Accountancy and consulting group BDP LLP said like-for-like sales increased by 1.9 percent for Brtiain's larger retailers in the five weeks that ended on 29 December thanks to a wave of online shopping and bargain hunting. The group's annual High Street Sales Tracker said online sales surged 30.9 percent from the same period last year while footfall on the weekend before Christmas was up 22 percent.