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* Scots to vote on independence on Sept. 18
* Nationalists say pound best option for independent state
* UK politicians to tell them they can't keep the currency
* UK government is stepping up campaign against a breakaway
By Andrew Osborn and Belinda Goldsmith
LONDON/KILMARNOCK, Feb 12 (Reuters) - Britain's mainpolitical parties will tell Scotland in coordinated statementsin coming days it cannot keep the pound if it votes forindependence, two people familiar with the matter said onWednesday.
Scottish nationalists accused British politicians ofbluffing and said they knew well any bid to withdraw the poundfrom Scotland would damage English business. They said it wouldmean Scotland would not shoulder any share of British debt.
The drive by the three main parties, the boldest attempt yetby the British political establishment to scuttle Scottishnationalists' independence bid, reflects a growing sense thatthe issue of what currency a breakaway Scotland would use couldbe decisive in swaying undecided voters.
Scots are due to vote on whether their oil-rich country of 5million becomes independent on Sept. 18, with polls showing justover one third favour independence. More assertive noises fromLondon may move some toying with the idea of independence tocaution but could also inflame some national feeling.
"I think telling Scotland what we can and cannot do from thesouth could upset some people but I will still vote againstindependence," said Sheila Lochhead, 65, a carer in the town ofKilmarnock, one of the areas of highest unemployment inScotland.
George Osborne, Britain's Conservative finance minister,will deliver the message in a speech on Thursday, taking aim atone of the central policies of the pro-independence ScottishNational Party (SNP) which argues a currency union with the UKwould be the only viable option.
Danny Alexander, Osborne's deputy and a Liberal Democrat, isexpected to reinforce that message on Thursday. Ed Balls, theopposition Labour party's spokesman on financial affairs, isexpected to say the same separately at an unspecified date.
The coordinated move leaves the SNP without an immediateplan B since it has built its case for independence around beingable to share the pound and has not disclosed a fallbackscenario. An independent Scotland would need to apply to jointhe European Union before it could ask to join the euro, and theSNP has so far shown no enthusiasm for the single currency.
Joining the euro or creating a new currency are seen as moreexpensive and potentially riskier alternatives.
The currency issue goes to the heart of what has become anincreasingly fractious debate as Scots say the economy is thesingle most important issue for them when it comes to choosingwhether to end their 307-year-old union with England.
"It's safe to say there's going to be movement on thecurrency issue," one person familiar with British governmentthinking told Reuters. Another source said the government wouldspell out its policy on the pound in clear and concrete terms.
A spokesman for the UK Treasury said he said he was unableto immediately comment on the matter.
'BULLYING TACTICS'
Scottish nationalists argue that Scotland's development hasbeen stunted by a British economy focused too strongly onLondon, and that an independent Scotland could flourish drawingon energy resources and technological enterprise.
The SNP said in a statement that the British government hadlost the argument against independence and was thereforeresorting to bullying tactics in panic.
It said it thought the shift in position on the pound was abluff and said it might not take on any share of UK debt in theevent of independence if it wasn't allowed to use the pound,something it has said before.
"Osborne's position is ... a bluff," said Nicola Sturgeon,Scotland's SNP Deputy First Minister.
"It would cost their own businesses hundreds of millions ofpounds a year," she said. "It would blow a massive hole in theirbalance of payments and it would leave them having to pick upthe entirety of UK debt."
Sturgeon suggested Britain would be powerless to stop anindependent Scotland using the pound.
"We have set out a reasonable case that we should continueto use our pound because it is ours as much as it is anybodyelse's," she said. "Neither George Osborne nor anyone else canstop Scotland using the pound."
Analysts say an independent Scotland would not need to askLondon for permission to use the pound but would then have thestatus of a country like Ecuador which uses the U.S. dollar buthas no say over Washington's monetary policy.
Royal Bank of Scotland, Lloyds Banking Group and other major financial institutions based inEdinburgh, have begun contingency planning in case ofindependence, Reuters reported on Friday.
Industry sources told Reuters on Tuesday a key part of thatplanning is what they will do in the event of a currency unionnot being agreed. RBS, Lloyds and Standard Life declined to comment.
Osborne has previously said it would be highly unlikely thatthe UK would be willing to let Scotland keep the pound, whileBank of England chief Mark Carney has said Scotland would haveto surrender some sovereignty in the event of independence and acurrency union.
Cameron said last week a Scottish vote for independencewould undermine Britain's global clout and imperil its financialand political stability.
