* Vodafone falls 4.2 pct to take most points off FTSE
* M&S falls after warning of website issues
* FTSE 100 down 0.4 pct
By Sudip Kar-Gupta
LONDON, May 20 (Reuters) - Sell-offs in mobile operator Vodafone and retailer Marks & Spencer dragged down Britain's blue-chip equity index on Tuesday, after both groups unveiled disappointing forecasts.
In mid-session trading, the benchmark FTSE 100 index was down by 0.4 percent or 27.90 points at 6,816.65 points.
Vodafone took the most points off the FTSE, falling 4.2 percent after the company said core earnings would fall in 2015 due to investment spending needed for its business. Analysts at Societe Generale said the company's results were lacklustre.
Clothing and food retailer Marks & Spencer also fell 3 percent after it warned that a new website would take four to six months to "settle in", affecting the performance of its general merchandise business in the three months to end-June.
The FTSE 100, which is up by around 1 percent since the start of 2014, hit its highest level in more than 14 years last week but has since slipped back.
"We're stalling up at these levels. I see a bit of a consolidation and then maybe the FTSE starting to drift lower over the next couple of weeks," said Hantec Markets analyst Richard Perry.
But Manoj Ladwa, head of trading at TJM Partners, saw positive support for the FTSE from the fact that it had held above the 6,800 point level, and expected the index to head towards 6,900 points.
"The FTSE is not showing signs of a sell-off from here," he said. (Additional reporting by Francesco Canepa; Editing by Catherine Evans)