UK's FTSE propped up by gains in M&S

* Marks & Spencer (Other OTC: MAKSF - news) shares surge after business update

* FTSE 100 up 0.1 pct

* Election uncertainty seen keeping FTSE in check near-term

By Sudip Kar-Gupta

LONDON, April 2 (Reuters) - Britain's top share index rose marginally on Thursday, helped by a surge in the shares of retailer Marks & Spencer after M&S posted its best non-food sales performance for nearly four years.

The blue-chip FTSE 100 index rose 0.1 percent to 6,813.14 points going into the middle of the trading session, with volumes relatively thin ahead of the Easter holiday break.

M&S rose 5.5 percent, making it the best-performing FTSE 100 stock in percentage terms.

M&S said sales of general merchandise, spanning clothing, footwear and homewares, at stores open for more than a year rose 0.7 percent in the 13 weeks to March 28, its fiscal fourth quarter.

The outcome was also the first time in 15 quarters that M&S had not posted a fall in non-food like-for-like sales, and full-year gross margin guidance for its food business was maintained at "up 10 to 30 basis points".

"The continued strong growth in food proved that quality mixed with competitive pricing can more than match the discounters and big food supermarkets," said London Capital (LSE: LCG.L - news) Group dealer Lewis Sturdy.

CONSTRUCTION CONFIDENCE

Traders added that UK construction data had also supported the FTSE 100.

While growth in Britain's construction industry slowed in March, confidence in the sector surged to a nine-year high.

Spreadex analyst Connor Campbell said the data was helping keep the FTSE afloat, even though uncertainty before the May 7 national election was likely to keep the index stuck in a tight range until the election result was known.

The FTSE hit a record high of 7,065.08 points last month. It has since slipped back but remains up 4 percent since the start of 2015.

Prime Minister David Cameron will face off against six political rivals in the first and only full televised debate ahead of the unusually close election.

Neither Cameron's Conservatives nor Ed Miliband's Labour Party have a clear lead in the polls, while a possible referendum on Britain's EU membership, promised by the Conservatives, is adding to nervousness among investors.

"The FTSE's natural predilection for the next month may be stagnation, as investors try and wait out the increasingly uncertain election run-in," said Campbell. (Additional reporting by Atul Prakash; Editing by Kevin Liffey)