The S&P/ASX 200 (Index: ^AXJO) (ASX:XJO) has given back more than half of the gains it made yesterday following the surprise rate cut by the RBA.

The main index is currently trading 1.2% lower at 5,286 points thanks to losses in the financial, energy and resources sectors.

Four of the worst performing shares today include:

BHP Billiton Limited (ASX: BHP)

BHP is the biggest drag on the market today with its shares plunging nearly 10% to $18.70. Today’s heavy falls follow the news that the mining giant and its partners are facing a $58.2 billion lawsuit from Brazilian prosecutors resulting from the Samarco dam disaster last year. BHP Billiton has not yet received formal notice of the claim, but investors are clearly concerned about the implications this could have on the company’s balance sheet. The miner has already committed a significant amount of funding for compensation and rehabilitation to those areas impacted by the disaster.

Shares of BHP have lost more than 43% over the past 12 months.

Woolworths Limited (ASX: WOW)

Woolworths is the other big name to fall heavily today with its shares losing more than 6.5% to $20.83. It comes after yesterday’s disappointing third quarter sales update and is followed today by a credit rating downgrade. Woolworths’ long term credit rating has been lowered by one notch to BBB from BBB+ by ratings agency Standard & Poor’s, with the agency citing market share losses and difficult trading conditions as the reasons for the downgrade. Although today’s credit downgrade may result in higher borrowing costs for Woolworths, I suspect today’s market reaction may have more to do with yesterday’s poor trading update.

Woolworths shares have lost more than 29% over the past 12 months.

Cover-More Group Ltd (ASX: CVO)

Shares of the travel insurance company have been hit hard today, falling more than 10% to an all time low of $1.207. Today’s fall comes on the back of a broker downgrade from Morgan Stanley and an uninspiring company update that was released yesterday. Despite announcing a new agreement with Flight Centre Travel Group Ltd (ASX: FLT) to provide travel insurance in the US, Cover-More also advised the market that group revenue growth for the third quarter was 4.7%. This was well below market expectations and investors will be eagerly awaiting further information when the company provides a more detailed update tomorrow.

Cover-More shares have lost 41% over the past 12 months.

Santos Ltd (ASX: STO)

The energy sector is underperforming today following an overnight fall in the oil price to below US$44 per barrel. Santos has been one of the worst performers in the sector with its shares falling by more than 6% to $4.30. The company also held its AGM today and one of the most interesting points to come out of the meeting was its aim is to be cash flow break-even at between US$35-40 per barrel on a portfolio basis. This doesn’t provide the company with a great deal of breathing space especially considering oil is now trading back below US$44 per barrel. Investors should therefore expect a high level of volatility from Santos shares until the oil price makes a significant recovery.

Santos shares have lost 50% over the past 12 months.

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Motley Fool contributor Christopher Georges has no position in any stocks mentioned. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.