Aside from Inveraray Castle, the estate operates a 200-unit caravan park.
Mr Nicol said: 'A key part of tourism is being able to tour. If you can't follow a particular route, you'll go somewhere else.
'People looking to invest in a caravan will visit perhaps three times. If they are diverted or sit in queues at the Rest for half an hour, they'll take their money elsewhere.'
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'With 200,000 sheep and 20,000 head of cattle going over that road, when there are delays with an extra hour of travel, from a welfare point of view that is intolerable.'
Stressing the industry's fragility, he added: 'All the milk from Kintyre's 27 dairy farms now goes over the Rest. When the CalMac Clyde ferry is off, all of Bute's milk has to use the A83 as well.'
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Loch Fyne Oysters not only relies on the Rest to bring goods in and take products out, but also to bring customers to the restaurant.
'A lot of our customers come from the Glasgow area so it badly affects us when the Rest is closed.
'We have to get product out to Larkhall and the airport to meet customer orders across the UK and abroad.
'It's really important we get fish and shellfish out as fresh as possible to fulfil our orders.'
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'We now find that squads travelling from the central belt won't come any more. We also find that other professionals are not keen to come to Argyll because they can't guarantee to get back at a particular time.
'It is affecting the building of houses in Argyll.'
Mr MacLeod added: 'It's a disgrace that the Rest has reached this stage. If it was a building site it would be shut down.
'Why are we running under a place where 100,000 tonnes could come down on top of us? There is going to be a serious accident there.'
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Argyll produces 10 per cent of the UK's softwood. Most of this goes through the central belt, so if the Rest is closed it's a disaster.
'At this week's prices, using the long diversion via Crianlarich it would add £55 per lorry. Timber is high volume, low margin so anything that increases costs is a problem.'
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