Police Scotland Chief Constable Sir Stephen House has defended the single force after a former SNP leader called for it to be broken up.

In a statement issued with Vic Emery, chairman of the Scottish Police Authority (SPA), Sir Stephen hit back at Gordon Wilson's claims that the single police force created in 2013 suffers from "delusions of grandeur" and should be federalised.

The joint statement prompted a furious response from the Liberal Democrats, who accused the Chief Constable and the SPA, the body responsible for holding the force to account, of acting as a "united mouthpiece" for the Scottish Government.

Mr Wilson called for "root and branch" reform, including the division of the service into four regional forces and for Holyrood's Justice Committee to take on the functions of the SPA.

Mr Emery and Sir Stephen said Police Scotland had delivered "an end to the postcode lotteries" of policing in areas such as domestic abuse and child protection and argued that policing in Scotland "has never been the subject of more scrutiny".

"Delivering a policing service that is responsive to local needs is the very heart of what Police Scotland was set up to do," they said.

The statement concluded: "As with all public services, there are continuous challenges. Since 2013, we have already saved the equivalent of three legacy force budgets.

"By improving efficiencies in our management structures alone we have saved more than 50 per cent of the costs of the former police force executive structures.

"We were clear from the start that this would be a long term journey of reform and change.

"We are only 30 months into that and we are learning and evolving our approach all the time.

"All journeys have points where it looks more tempting to turn back than go on.

"That would risk us failing to realise many of the benefits of a single service that are as yet only partly realised. It is a temptation we should resist."

Liberal Democrat justice spokeswoman Alison McInnes described the joint statement as "astonishing" and said it indicated that the force and the SPA were "unwilling to learn lessons".

She said: "The Chief Constable and the chairman of the SPA are acting as spokespeople for the Scottish Government when they should be fixing the problems created by the reforms.

Labour justice spokesman Graeme Pearson said: "Had policing truly been delivered these past three years that was responsive to local needs at its very heart we would not face the mess we have now."