The British Transport Police (BTP) has warned the "seamless" policing that currently covers the UK railways could be put at risk if the force north of the border is merged with Police Scotland.

Transport police chiefs are also concerned the specialist knowledge of dedicated officers on the railways could be lost if the plans go ahead.

The force raised concerns ahead of a meeting of Holyrood's Justice Committee, where BTP Chief Constable Paul Crowther will address MSPs.

A written submission makes clear the" current specialist approach of BTP has been developed over many years, underpinned by a deep and clear understanding of the unique requirements of the railway".

The BTP said: "Analysis has shown that dealing with incidents can take significantly longer if officers inexperienced in railway policing are the first responders.

"BTP's analysis reveals that offences involving cable theft take on average 33% longer to manage whilst fatal incidents can take almost 50% longer."

The railways are currently policed "in a seamless way which avoids the need for officers to disembark" when travelling across the border between Scotland and England, the submission said.

This protects the travelling public, the BTP said, and also prevents disruption to operators of cross-border services, including "the regular policing of football supporters travelling between cities and towns in Scotland in to the north of England and vice versa".

The force also stated: "Specialist train services including nuclear trains, MoD trains and the Royal Train are also currently policed in a continuous manner by BTP operations that consider the implications of the end-to-end route.

"Introducing new arrangements that necessitate the handing-over of command for any of these services will need to be carefully developed."

Meanwhile, BTP officers "have taken the decision to join a specialist police service and are understandably proud of their 'expert' role and the work they do", the submission said.

While in the short term the transfer of these officers to Police Scotland would "help retain specialist knowledge", the force warned "there is the possibility that despite the best of intentions, this might not be sustainable in the longer-term".

Ministers at Holyrood have put forward legislation that, if passed, would give power over railway policing to the force and the Scottish Police Authority (SPA) watchdog.

Tory justice spokesman Douglas Ross said the "extraordinary" arguments from the BTP "should leave the SNP in no doubt about why it should not pursue this merger".

The Conservative said: "The Scottish Government should heed these warnings and shelve these unnecessary and unpopular plans immediately."

Police Scotland said the lessons learned from its own creation - when eight regional police forces were brought together to create a national body - would be applied to the integration of BTP.

The force said: "It should also be noted that during the early life of Police Scotland a highly professional level of service continued to be provided to the communities of Scotland.

"During the integration of BTP we will aim to ensure this is replicated across Police Scotland and the railway network."

Police Scotland's submission highlighted its intention to "retain the current specialist skills and knowledge built up by BTP officers to ensure a smooth transition into Police Scotland", adding a training programme would be developed to "upskill" current frontline Police Scotland officers.

The force argued: "Devolution of railway policing will have no detrimental impact on cross-border security arrangements.

"BTP and Police Scotland currently work together on a number of cross-border operations and this close working relationship would be expected to continue with BTP colleagues south of the border, following April 1 2019."

Liberal Democrat justice spokesman Liam McArthur said: "Evidence from the British Transport Police issues a clear and stark warning: specialists are required to provide the consistent levels of service that passengers and railway staff are accustomed to and expect.

"The Scottish Government must show that it has learned from the calamitous centralisation of Police Scotland. That starts with explaining how this expertise will be protected and ring-fenced in future."

A Scottish Government spokesman said: "As Police Scotland have made clear, specialist railway policing expertise and capacity will be maintained and protected within the broader structure of Police Scotland, with improved access to wider support facilities and specialist equipment, providing an enhanced service provision to the rail industry and travelling public.

"Devolution of BTP's functions was recommended by the Smith Commission, reached through cross-party agreement and integration will also ensure railway policing is fully accountable to the Scottish Parliament."