The leader of Britain's biggest transport union has condemned plans to put Police Scotland in charge of policing railways in Scotland.

RMT General Secretary Mick Cash said the SNP Government's plans to merge the British Transport Police (BTP) with the national force were "ill-conceived and dangerous".

Justice Secretary Michael Matheson last week confirmed that BTP's Scottish branch would be consumed in to Police Scotland by the end of next year.

It came after the Smith Commission gave Holyrood oversight over railway policing.

BTP leaders in England had hoped their force could continue to offer a discrete service - although Scottish sources in the force have long argued they could prove a valued asset to Police Scotland.

RMT has echoed concerns from unionist parties who believe that the merger could mean the loss of specialist skills. Labour, the Conservatives and the Liberal Democrats have accused the SNP of making a "political point" over the force because of its UK-wide remit.

The BTP has a focus on security, with routine criminality is extremely low on the railways.

Mr Cash, whose union supported independence at last year's referendum said: "It is sheer arrogance on the part of the SNP Government that they are ignoring the advice of the BTP and those who work in the railway industry and are forging ahead with plans to merge this important and distinctive policing operation into the wider force.

"Skills and expertise in dealing with the specialist policing needs on the railways would be lost for ever and would result in an inferior policing service which would impact on staff and passengers alike.

"RMT stands alongside our sister rail unions in opposing these ill-conceived and dangerous plans."

There are more than 200 BTP officers in Scotland.

They are subject to English pay and conditions, which are - broadly - not as good as those in Police Scotland.

Crucially, BTP officers in Scotland operate under Scots Law and therefore share many practices with Police Scotland colleagues rather than their UK counterparts.

The transport force is funded by railway operators, which are understood to be unhappy with the proposal for a merger.

However, some police sources argue that cash from Scotrail, the main train firm north of the border, is effectively subsidising policing in the rest of the UK. One said: "Financially, this is a no-brainer."

BTP's Scottish branch, which has a headquarters in Glasgow and hubs at major railway stations, would most likely simply become a specialist division of Police Scotland.

The transport force has considerable expertise in counter-terrorism and protecting infrastructure from metal thefts, both of which could help wider Scottish policing. Sir Stephen House is understood to want BTP officers to continue doing what they do now after the merger.

Mr Matheson said he believed that BTP would be a good "fit" with Police Scotland.

The government has said it will meet with operators and other parties. BTP chief constable Paul Crowther, said passenger safety would remain the "top priority" as talks over the details of the amalgamation continue.