* UK Q4 underlying sales down 4.2 percent, beating forecasts
* Food sales improving, holding market share in clothes
* Reiterates previous margin, cost and capex guidance
* Outlook remains "uncertain"
(Adds detail, background)
By James Davey and Mark Potter
LONDON, March 31 (Reuters) - Landmark British retailer Marks & Spencer posted a smaller-than-expected fall in fourth-quarter underlying sales, saying it was turning around its troubled food business and holding market share in clothing.
The 125-year-old firm, which serves more than 21 million Britons a week from over 600 stores, said on Tuesday the trading outlook remained "uncertain", but kept its previous guidance on profit margins, operating costs and capital spending.
Sales at UK stores open at least a year fell 4.2 percent in the 13 weeks to March 28, improving on a 7.1 percent drop in the third quarter and beating analysts' forecasts for a decline of between 6.5 and 7.5 percent, according to a company poll.
Like-for-like UK general merchandise sales, comprising clothing and homewares, fell 4.8 percent after an 8.9 percent drop in the third quarter, while underlying food sales were down 3.7 percent, having fallen 5.2 percent the previous quarter.
Adjusting for the timing of Easter would add about 0.7 percentage points to both measures, Marks & Spencer (M&S) said.
M&S, Britain's biggest clothing retailer, has been hit hard by a consumer downturn, exacerbated by self-confessed mistakes, particularly in its upmarket food business.
The downturn has added to pressure on Executive Chairman Stuart Rose, who was already under fire from some investors for combining the roles of chief executive and chairman, against corporate governance best practice.
"Our customers are responding positively to the actions we have taken," Rose said in a statement, highlighting the group's popular "Dine In" food promotions and the success of its new Portfolio womenswear range.
M&S shares have underperformed the DJ Stoxx European retail index by 14 percent over the past 12 months, but have been the FTSE-100's strongest performing retail stock this year on hopes of an eventual recovery.
The shares closed at 264.50 pence on Monday, valuing the business at about 4.2 billion pounds ($6 billion). (Editing by James Davey/Will Waterman) ($1=.7052 Pound)