Police Scotland could be facing a staffing crisis as senior officers insisted they need "boots on the ground" throughout 2020 while watchdogs warned budgetary pressures mean current officer numbers are "not sustainable".

In a year in which Scottish officers will have to police the global COP26 climate change summit in Glasgow, Euro 2020 football matches, major environmental protests, and other events, Deputy Chief Constable Will Kerr said the force will need to deploy "significant numbers of police officers effectively every month during the rest of this calendar year".

Speaking about the demands Police Scotland is facing, he said: "Frankly and bluntly, 2020 is going to be about boots on the ground, when you distill it down in its purest policing sense.

"We're going to have environmental protests that are ongoing on a weekly basis now, the Euro 2020 coming up in June and the preparation for those, some of the ongoing issues and protest activity associated with the political debate about an independence referendum, COP26, and parades."

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Mr Kerr told the Scottish Police Authority (SPA): "All of those are very resource-intensive if we are going to maintain the safety and security of Scotland."

At the same meeting, SPA vice chair David Crichton warned the current number of police officers in Scotland is "not sustainable" as a result of a "structural deficit" in the force's budget.

He said "difficult choices" will need to be made by the force and by Scottish Government ministers.

He told the meeting in Edinburgh: "We at the authority have been raising our concerns about financial sustainability consistently over the last four or five months.

"The chief constable's been raising it and Audit Scotland has raised it as well.

"There is a structural deficit in the policing budget. It's simple arithmetic, it's not complicated mathematics, it's simple arithmetic.

"With almost 90% of the budget allocated to officer and staff costs, it does mean that difficult choices are going to have to be made over the next weeks and months - difficult choices by Government, by the authority and by Police Scotland.

"Frankly, current officer numbers are not sustainable within the existing budget so something has to change on that front.

"The deficit is simply going to continue to increase if something does not change."

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Official figures showed there were 17,256 full-time equivalent police officers in Scotland at the end of September 2019.

Police Scotland Chief Constable Iain Livingstone told the SPA meeting there had been an increase in the demands on officers last year.

He said there was an almost 20% rise - from around 1,500 to 1,800 - in the number of loyalist and republican marches in Scotland, as well as a higher number of spontaneous protests such as those by Extinction Rebellion.

"My priority is to build a sustainable police service that has the right mix of police officers and police staff that operates within its budget," he said.

"But at the moment there's an operational imperative, I sense an element of political imperative, to maintain officer numbers and the challenge for us is showing the value that having a strong police service provides and at this stage, making a case for further investment."

A Scottish Government spokesman said: "Despite constraints on Scotland's public services through a decade of UK austerity, we have worked with Police Scotland and the SPA to maintain and improve policing services, including providing significantly more officers than at any time before 2007 - at the same time as numbers in England have declined by more than 19,000.

"Budget discussions are ongoing and we also continue to press the UK Government to repay the £125 million VAT paid before the Treasury reversed the policy.

"However, we will continue to ensure Scotland's police service is well funded.

"As ministers have made clear since Police Scotland and the SPA published their 2026 strategy, we expect officer numbers to remain significantly above the level inherited in 2007."