Despite Marks & Spencer's disappointing Christmas trading update on Thursday, brokers Investec and The Share Centre have retained their positive recommendations on the stock.While the food division continues to outperform the industry with like-for-like (LFL) in the third quarter ended 27 December up 0.1%, general merchandise (GM) LFL sales fared much worse than expected after falling 5.8%.Analyst Kate Calvert from Investec trimmed her profit forecasts for the company following the statement, but said that the worse-than-expected performance was partly offset by improvement in cost guidance for the full year from +3.5% to +2%.The company also maintained its margin forecasts for both food and GM and said that all other guidance remains unchanged."Given the number of issues in different areas of the business, it is perhaps remarkable that full-year profit guidance was maintained," said The Share Centre's research analyst Ian Forrest, who kept his 'buy' rating on the retailer."There is still work to do in the group's general merchandise division, however food sales remain very strong. The falling oil price, which should increase consumers' disposable incomes, could benefit the company. In volatile markets the prospective dividend yield of 4% is also attractive for investors," he said.Investec's Calvert also reiterated a 'buy' on M&S, saying that the stock's current valuation does not reflect the company's "gross margin opportunity and shift to cash generation".She said: "The unseasonal weather and online own goal yet again overshadows reasonable recovery progress elsewhere. Gross margin gains, efficiencies and better ranges are all visible and will feature in 2015."The stock was down 4.6% at 441.7p by 11:02.