Flybe and Eastern Airways today announced they would compete with Loganair on five Highlands and Islands air routes.
The dramatic move follows the breakdown of a partnership between Flybe and Loganair, which ends in August.
Loganair, which currently operates flights for Flybe, said it was "extremely disappointed" but it was "up for the fight" when it goes it alone in September.
Eastern will operate flights for Flybe between Sumburgh in Shetland and Edinburgh, Glasgow and Aberdeen, between Stornoway in Lewis and Glasgow, and between Kirkwall in Orkney and Aberdeen, from the same month.
Flybe pledged shorter journeys using faster aircraft on the routes, which are among the busiest it currently operates with Loganair.
It said one-way fares would start at £49.46.
The move follows Flybe announcing last November it had "terminated" the franchise deal with Loganair, which has operated since 2008.
Loganair managing director Jonathan Hinkles told The Scotsman it may hit back by competing with Flybe on some cross-Border routes.
BACKGROUND: Loganair to set a course into the unknown http://www.scotsman.com/news/transport/alastair-dalton-loganair-to-set-a-new-course-into-the-unknown-1-4312526
Aviation consultant John Strickland told The Scotsman: "This brings a whole new level of competition to a number of important Scottish domestic routes.
"The key to success will depend on whether the market grows to allow profitability for both operators or whether it a simply develops into a price battle for existing traffic."
Flybe said: "We believe these new services provide an excellent proposition for our Scottish customers, increase market competition, and with the links into the broader Flybe network, provide seamless connectivity to a broad range of UK and international destinations.
"We are confident customers will value the benefits provided that include larger, faster aircraft."
But Mr Hinkles said: "We are extremely disappointed and surprised at this announcement.
"We have been negotiating in good faith with senior Flybe staff about a continuing relationship to ensure customers could connect from all of Loganair’s extensive Highlands and Islands network to the rest of the UK.
"Many of these connections will now be lost as a result of Flybe’s decision.”
"Flybe is intent on challenging us head-to-head on a few of our traditional and established routes that make up our extensive network in Scotland, but we are Scotland's Airline, and I can promise them that we are up for the fight.
"We’ve invested a huge amount in the infrastructure – engineers, ground handling, support staff – to provide our Highlands and Islands services and we’re here to stay.”
"Although we've been flying as a Flybe franchisee on these routes for the last eight years, we have been serving these communities for over 50 years.
"We will continue to offer best value and service to our customers including a free checked bag and no credit card payment surcharges on all routes.
"Flybe’s extra charges add at least £44 return for a bag and £6.95 surcharge to pay by credit card – a huge extra cost for passengers."
"As we get closer to the date of our transition from franchisee to flying as a solo airline on September 1 with a continuing codeshare partnership with British Airways for connections to London and worldwide and a new relationship with bmi Regional for UK and Scandinavia connections, we are all very much looking forward to providing the highest standards of safety, punctuality and service synonymous with the Loganair name."
Flybe said it had signed "heads of terms" with Eastern Airways to establish a new alliance, in the first step "towards finalising a five-year franchise partnership" under which Eastern will operate its scheduled route network under the Flybe brand.
Chief executive Christine Ourmieres-Widener said: “This alliance with Eastern Airways is an excellent opportunity to strengthen the Flybe network.
"We already operate flights at a number of common airports and this will enable passengers to connect to a greater number of destinations, across the UK and into Europe."
The Scottish Government said it was a positive move.
A spokesman for its Transport Scotland agency said: "Fast, frequent connections between our islands and the mainland are essential.
"We welcome this commitment by Flybe and Eastern Airways, which will greatly increase capacity and choice on some of the most popular intra-Scotland routes.”