The local authority was expecting to have to tackle a funding gap of more than £21m, but that figure has been revised up to £40m.
The council said the figure was reviewed following last week's UK government Autumn Statement.
Senior councillors said they wanted to protect education from cuts and avoid compulsory redundancies.
Council leader Margaret Davidson said the Autumn Statement had included a 4-5% cut to Scotland's budget and this would filter down to local authorities in terms of less funding from the Scottish government.
She told BBC Alba that trying to find savings of £21m, which was on top of £10m of cuts already agreed for 2016-17, had been "bad enough".
She added: "Now we are being told the gap could be up to £40m, almost double what we had been expecting."
Ms Davidson said Highland Council would be calling for an end to the council tax freeze, as it has done in the past, to allow it to generate more revenue.
Budget leader Bill Fernie described the savings local authorities were having to make as the hardest since World War Two.
Last week, Highland Council said it was looking at closing all its offices and schools from lunchtime on Fridays to help it save money.
Views have been sought from the public on the plan and other ideas to help the council tackle a hole in its budget.
Reducing the working and school week to 4.5 days would result in significant savings, the council has said.

The Staffordshire Regiment Association said it paid Â£64,000 to buy a 10-year lease on The King's Head in Lichfield.
The regiment was founded in 1705 when Col Luke Lillingston raised a force that later became the Staffords.
Capt Dave Donnelly said: "It's brilliant. We believe we have bought a piece of history."
He said the purchase was prompted by a "mad idea" by ex-warrant officer Derek Watts, from Wolverhampton, who went on social media to drum up support.
Within a fortnight, he had managed to raise enough pledges to buy the pub.
"We have reunions in the pub annually, so we didn't want it becoming a bed and breakfast or anything like that," said Capt Donnelly.
"The support to get this off the ground has been immense. We don't know how many associations run pubs, but what is unique is that we were actually founded there."
The pub will reopen - following a refurbishment - on March 25, 312 years to the day from when the regiment was formed.
It will serve two ales created in honour of the regiment - one named Col Lillingston and the other named Watchman, after the association's mascot - and will be managed by a former regiment man.
It will be opened by Lichfield's Lord Mayor and a descendent of Col Lillingston. The association will march from Lichfield Cathedral to the pub and will play the Last Post.

Jamie Hall, 22, made no plea or declaration during a private hearing at the city's sheriff court on Monday.
Prosecutors allege he carried out the sexual assault in the city's Spring Gardens.
Mr Hall, from Edinburgh, was remanded in custody ahead of his next court appearance.

13 October 2016 Last updated at 00:23 BST
Many analysts think Sony's kit will outsell rival headsets from HTC and Facebook's Oculus division because of its lower price and the fact that more than 43 million PS4 consoles have been shipped.
Even so, they add, it is likely to remain a niche product for some time to come.
Strategy Analytics forecasts that only 3% of Brits will own a VR headset this year and the PlayStation model will account for just 7% of that number. The bulk, it says, will instead own more basic "shells" into which they can slot a smartphone.
BBC Click's Marc Cieslak put the PS VR to the test and had reservations of his own.
More at BBC.com/Click and @BBCClick.
Highland Council has said it must save more money in 2016-17 than it had previously thought.
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Veterans of a historical Army regiment have bought the lease on the pub where it was founded.
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The PlayStation's virtual reality headset is making its way into gamers' homes - the last of this year's three big VR hardware releases.