* FTSE 100 closes down 1 pct at 6,557.52 points
* Sainsbury cuts sales forecast and puts dividend on review
* Regulators probe Tesco
* FTSE ends at lowest closing level since mid-April
By Sudip Kar-Gupta
LONDON, Oct 1 (Reuters) - Britain's top equity index fell to its lowest closing level in nearly six months on Wednesday, as a new slump in the shares of supermarket retailers hit the market.
Sainsbury fell 7 percent after the supermarket operator cut its annual sales forecast and put its dividend under review. Rival Tesco weakened 3.2 percent after saying it was under investigation by regulators.
The investigation into Tesco comes after an accounting scandal that has wiped billions off Tesco's market capitalisation over the past week and a half.
"Dire news from the supermarket sector shows that investors should still be looking elsewhere. Any rally in supermarket shares will be an invitation for fresh selling," said IG market analyst Chris Beauchamp.
The blue-chip FTSE 100 index fell 1 percent, or 65.20 points, to 6,557.52 points - its lowest closing level since April 15.
"We would be concerned if the FTSE fell below 6,500 or 6,425 points, but we're not ready to jump in with the bears just yet," said Joe Neighbour, trading analyst at Central Markets.
Terry Torrison, managing director at Monaco-based McLaren Securities, had a more negative view.
"I'm in the bearish camp at the moment. We've had a good run up, but it's time to consolidate. I would prefer to sell the market at the moment," he said.
The FTSE is down by around 3 percent since the start of 2014, underperforming a gain of around 4 percent on the broader pan-European FTSEurofirst 300 index.
The index is also some 5 percent below its 2014 peak of 6,904.86 points reached last month, which marked the FTSE's highest level since early 2000. (Additional reporting by Francesco Canepa; Editing by Hugh Lawson)