LONDON (Reuters) - The leading share index slipped by midday on Wednesday giving up initial strength provided by rising mining stocks, while banking stocks remained weaker.

At 11:54 a.m., the FTSE 100 <.FTSE> was down 20.1 points, 0.3 percent at 6014.6 points.

"The whole story is oil and mining. Commodity prices are responding largely to the weaker dollar," said Investec (LSE: INVP.L - news) 's Roger Cursley.

"It (Frankfurt: A0MLX5 - news) feels like commodity prices have a lot of momentum. Away from that everything is quite red. The markets are not reacting that well to the RBS (LSE: RBS.L - news) rights issue. There is a concern that the economics outlook will put pressure on earnings," he said.

"Today we've had weakness in travel companies and bus companies and it's everything to do with the oil price."

The mining sector stood firmer across the board. BHP Billiton (LSE: BLT.L - news) <BLT.T> rose 1 percent. The world's largest mining group reported an 8 percent drop in third-quarter output of copper, its top revenue earner.

Lonmin (LSE: LMI.L - news) <LMI.L> rose 2.8 percent after the world's third biggest platinum producer posted an 8.3 percent rise in second-quarter platinum sales, but again cut its full-year sales target following power problems in South Africa.

Rio Tinto (Frankfurt: 855018 - news) <RIO.L>, Kazakhmys (LSE: KAZ.L - news) <KAZ.L>, Xstrata <XTA.L>, Anglo American (LSE: AAL.L - news) <AAL.L> and Antofagasta (LSE: ANTO.L - news) <ANTO.L> were up between 0.4 and 2.1 percent.

Banks weakened with Royal Bank of Scotland (LSE: 90ID.L - news) <RBS.L> leading the way losing 6.2 percent following its unveiling of a large rights issue on Tuesday. Barclays (LSE: BARC.L - news) <BARC.L> shed 3 percent, HBOS <HBOS.L> lost 5.3 percent, while Lloyds TSB <LLOY.L> slipped by more than 4 percent.

The travel and leisure industry fell amid strength in world oil prices. Thomas Cook (LSE: TCG.L - news) <TGL.L>, FirstGroup (LSE: FGP.L - news) <FGP.L>, British Airways (LSE: BAY.L - news) <BAY.L) and Tui Travel (LSE: TT.L - news) <TT.L> all fell by around 5 percent.

GlaxoSmithKline (LSE: GSK.L - news) <GSK.L> stood 0.3 percent higher ahead of the release of its first-quarter results. The drugmaker said it had agreed to buy biotechnology company Sirtris Pharmaceuticals <SIRT.O> for $720 million in cash.

Oil stocks stood slightly firmer as the U.S. crude <CLcl> contract for June traded shy of $118 a barrel. BP <BP.L>, Tullow Oil (Dublin: TQW.IR - news) <TLW.L> and Royal Dutch Shell (Amsterdam: RDSA.AS - news) <RDSa.L> gained up to 3 percent.

A source familiar with the matter said Shell (LSE: RDSB.L - news) was cutting hundreds of jobs in Aberdeen, as it sells oil fields in the declining North Sea.

Alliance and Leicester (LSE: AL.L - news) <ALLL.L> traded without the rights of dividend and slumped more than 10 percent. Other companies trading without dividend were Centrica (LSE: CNA.L - news) <CNA.L> and Tesco (LSE: TSCO.L - news) <TSCO.L> which slipped 4.5 and 1.6 percent respectively.

Also, Scottish & Newcastle <SCTN.L> was 0.3 percent higher. Britain's biggest brewer is set to leave the FTSE 100 next Monday following its acquisition by Heineken (Amsterdam: HEIA.AS - news) <HEIN.AS> and Carlsberg <CARLb.CO>.

Among mid-caps, consumer lender Cattles (LSE: CTT.L - news) <CTT.L> climbed 7.5 percent after it said it was raising 200 million pounds in a rights issue to support its application for a banking license and diversify its sources of funding.

(Editing by Elaine Hardcastle)