Security for the COP26 climate summit in Glasgow this year could cost "several hundred million pounds", a new report suggests.
Papers to go before the Scottish Police Authority (SPA) board this week have outlined the potential cost of the event, which will bring heads of state from around the world to Scotland in November.
The papers say "dialogue remains ongoing" with the UK Government as to who will foot the bill for security.
The Scottish Government has previously said Westminster should pick up the tab, with Justice Secretary Humza Yousaf claiming the UK Government should fund the entire security provision.
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It will be the largest summit the UK has held, with up to 200 world leaders expected for the final weekend of talks and around 30,000 delegates in total.
According to the papers, "detailed financial planning" is under way, with discussions between the the police authority and the Cabinet Office in Westminster ongoing.
In September - when Glasgow was announced as the host city for the summit - Police Scotland said the costs to the force would be in the "tens of millions", while October estimates placed the cost at £100 million.
It is not clear what has caused the increase in expected costs - but the new estimate has used other events, such as the Nato Summit in Wales in 2014, as a baseline for costings.
READ MORE: Boris Johnson's letter to Nicola Sturgeon rejecting indyref2 demand - in full The report said: "Taking into consideration the planning assumptions and based on previous major summits/conferences (e.g. Nato Summit Wales 2014), the initial costings demonstrate that the event will cost potentially several hundred million pounds.
"Detailed financial planning is being developed and dialogue remains ongoing with the Cabinet Office relative to the cost recovery model that will be utilised."
The report also outlines plans for police officers from other forces to be sent to Scotland to aid the force in what is described as "undoubtedly the largest mass mobilisation of police officers in the UK", adding it will present "extensive logistical challenges".
The Scottish Event Campus will play host to the event, being handed over to the United Nations for the summit's entirety.