The full cost of merging the British Transport Police with Police Scotland has not been assessed, according to a watchdog.

MSPs voted to combine the BTP with the national force in June despite widespread opposition.

The Scottish Government claims it will make railway policing more accountable.

A new report from Her Majesty's Inspectorate of Constabulary in Scotland (HMICS) highlights a series of unanswered questions.

Among them is how much the merger will cost and who will pay for it.

HMICS warned: "The full costs associated with the transfer of railway policing in Scotland have not yet been assessed and there is uncertainty among stakeholders as to who will pay these costs."

The £21m annual cost of running the BTP in Scotland - a bill expected to rise to £24m by 2022 - is paid by the country's three main rail operators.

Abellio ScotRail, Serco Caledonian and Network Rail are in turn supported by franchise payments and subsidies from the Scottish Government.

Industry representatives told HMICS they do not expect to pay for the integration.

The financial impact on railway policing in England and Wales has also not been fully assessed, HMICS said.

It noted that "little thought" had been given to how authorities would measure the success or failure of the merger.

Meanwhile, a report published on Friday by Scotland's auditor general criticised the spending practices of the Scottish Police Authority (SPA), Police Scotland's parent body.

It revealed that the SPA overspent its budget by £16.9m last year and that a senior police officer was paid £67,000 to move house.