An academic has resigned from an inquiry into the abuse of children in care, citing "interference" from the Scottish Government.

Professor Michael Lamb was a panel member of the Scottish Child Abuse Inquiry, set up to focus on allegations of abuse in formal institutional care settings.

Campaigners representing victims of childhood abuse said the scope of the inquiry should have been widened to include abuse at religious organisations and children's groups such as the Scouts, and called for clarity on support for survivors.

Prof Lamb sent his letter to deputy first minister John Swinney on Tuesday, saying there had been "repeated threats to the inquiry's independence".

However the Scottish Government has said it "entirely rejects" his claims.

The academic was appointed in October last year as one of the two panel members to support chairwoman Susan O'Brien QC.

Prof Lamb, of Cambridge University, is an award-winning academic who headed a research unit on child welfare in Washington DC for 17 years.

In his letter, he said: "It has become increasingly clear over the last nine months that the panel cannot act independently and that the Scottish Government intends to continue interfering in ways large and small, directly and indirectly.

"Continuing interference threatens to prevent the Inquiry from investigating thoroughly and taking robust evidence of the highest quality."

He added: "Crucially, its fact-finding should not be constrained or micromanaged by one of the bodies whose actions or failures to act may ultimately be criticised.

"Repeated threats to the inquiry's independence have undermined the panel's freedom to address the terms of reference and have doomed the inquiry before the first witness has been heard.

"The Scottish Government has delayed or prevented the appointment of crucial members of staff for prolonged periods of time while its officials have questioned the decisions made by the supposedly independent inquiry.

"This continuing pattern leaves me with no option but to resign. I cannot in good conscience give credibility and credence to an inquiry that is so profoundly limited in its independence and freedom to explore the troubled history of abuse in Scottish institutional care."

Campaigners had also called for a solution to remove the time bar on pursuing civil cases on historic child abuse.

A spokesman for child abuse survivors group Incas said: "This is a devastating indictment of the Scottish Government. Any sort of interference must result in resignation from office.

"This will have major detrimental effect on the confidence of survivors.

"Mr Swinney must make an urgent statement to Parliament . We need to know, who what and why. The Government needs to come clean."

A spokeswoman for the Scottish Government said: "We entirely reject Prof Lamb's comments about the Scottish Government.

"The Scottish Government has a clear obligation to fulfil its responsibilities within the requirements of The Inquiries Act 2005 and other relevant legislation.

"Our primary focus remains on supporting the successful operation of the independent statutory Inquiry.

"Ministers are grateful to Prof Lamb for his work."

The spokeswoman added: "The deputy first minister has also written to survivors and their representatives about Prof Lamb's departure and assured them that his primary objective is to ensure that this does not impact on the progress that the independent Inquiry has been making."

Scottish Labour said Prof Lamb's resignation was a "disgrace."

The party's education spokesman Iain Gray said: "For months now, Scottish Labour has warned that the Scottish Child Abuse Inquiry was close to losing the confidence of survivors.

"Now it has lost the confidence of one of the two inquiry panel members to the extent that he has resigned.

"Professor Lamb is absolutely clear that he is resigning because of SNP Government interference, compromising the independence of the inquiry and preventing it doing its job.

"SNP ministers have refused to listen to us, or to survivors, but surely they cannot ignore this. John Swinney must act now, halt the inquiry, investigate, and relaunch it in line with survivor demands and with new guarantees of independence from interference.

"He should make an emergency statement to parliament before recess to explain how he intends to fix this.

"However he must start by apologising to survivors, because this is a disgrace, and the SNP Government has let them down again and again."