A full merger of Police Scotland with the British Transport Police north of the border "will not be possible" by the April 2019 deadline, a senior officer has said.

Chief Superintendent John McBride said in a statement to British Transport Police (BTP) staff that Government officials will be informed at a meeting on January 30.

He said Police Scotland has put forward an alternative option of "operational integration" which means the single force taking over railway policing and all BTP staff but relying on BTP for support in areas such as technology, funding and data access.

His statement on BTP intranet said: "BTP has planned for full integration. The alternative proposal presents real challenges to deliver the outcomes of the project."

He added that transferring officers and staff from BTP to Police Scotland with their terms and conditions protected as agreed, is "proving a challenging and hugely complicated piece of work".

BTP Deputy Chief Constable Adrian Hanstock, who previously told MSPs he would prefer the creation of Transport Police Scotland than the merger, said in a internal statement: "We are at a very important stage in preparation for the future of railway policing in Scotland and I am pleased to see that those involved in the decision making process have listened very carefully to what we've said."

He said the decision means "informed proposals" can be put to the joint board overseeing the integration.

Holyrood passed legislation to subsume BTP in Scotland into the single force in June, despite widespread opposition, with detractors raising safety concerns and the government claiming it would make transport policing more accountable.

Scotland's acting top police officer, Deputy Chief Constable Iain Livingstone, told MSPs on the Justice Committee on Tuesday it had become “absolutely clear” that merger issues would not be rectified by the official deadline of 1 April next year.

The Herald: Pictured is Iain Livingstone (Deputy Chief Constable) speaaking at a press conference at Glasgow Central Mosque re. Stop Islamophobic Hate Crime and Unity Against Terror....   Photograph by Colin Mearns.20 November 2015.

He said the force had identified problems with the integration of IT systems, officers’ terms and conditions and pensions and would not be drawn on a date when full integration would occur.

Asked by  Scottish Conservative justice spokesman Liam Kerr when the problems might be resolved, he replied: “I can’t answer that specifically. “But I’m absolutely clear that those issues have such a level of complexity that they won’t be resolved by 1 April 2019.”

Tory MSP Maurice Corry told Mr Livingstone he had spoken to police officers in his area who were “really quite concerned” about the merger.

Mr Livingstone replied: “I share your concern. What’s clear and what has become clear is that this is not a merger of like with like.

“I can’t give you comfort that we’ve resolved the issues but I can give you an assurance that we’re working extremely hard and we do recognise the challenges.”

Scottish Conservatives plan to hold a debate at the Scottish Parliament on Thursday calling for the calling for the merger to be paused, citing "leadership challenges facing police Scotland" and the importance of railway policing in responding to terrorism.