The Scottish Child Abuse Inquiry is to be extended beyond its initial four-year remit after its chair told the Scottish Government the job was not possible in that time frame.

Deputy first minister John Swinney is to announce an extension of Scotland's biggest every public inquiry this afternoon after Lady Smith wrote to him a week ago requesting a change to its terms of reference.

The SCAI, which has so far cost £14million, was initially asked to report to ministers within four years. This would have meant it concluded by October 2019. 

However, according to a letter sent out to victims today "Lady Smith has informed Mr Swinney that it will not be possible to report in that timescale due to the very wide remit of the Inquiry covering from living memory to 2014 and the very large number of institutions providing care in the range of settings covered."

The remit of the inquiry is vast - taking in not just physical and sexual abuse but emotional and "spiritual" abuse too. It covers residential care homes run by the state, religious organisations and charities and in foster care. And it is empowered to investigate any events in living memory.

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The letter said Mr Swinney had agreed that "it is of the utmost importance that the Inquiry is given the time to investigate its remit thoroughly and properly"

It is understood that the minister wants to enable the Inquiry to hear from as many survivors as possible and has therefore agreed to publish amended terms of reference for the inquiry this afternoon.

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David Whelan, of the abuse survivor group Former Boys and Girls Abused of Quarriers Homes said he welcomed the news. "IF it is extended because the inquiry requires more time to investigate the issues and investigate them properly and thoroughly, then we welcome that" he said. "I would be concerned that it is not open ended."

Abuse campaigner Dave Sharp added: "This shows the inquiry is working and this is an opportunity to encourage as many people as possible to come forward."