Police officers in Scotland are being told to abandon proactive investigations into drug dealers in case they require overtime, a rank-and-file whistle-blower has claimed.

He told the Scottish Police Federation he risks "career suicide" by speaking out about the constraints officers feel as a result of the Police Scotland overtime ban.

The force is facing a £21 million shortfall in running costs this year, dwarfing the £18 million overspend last year, despite an £18 million budget boost from the Scottish Government for 2016/17.

Calum Steele, SPF general secretary, published a redacted copy of the email he received on Twitter.

It states: "You may already be aware that officers in [redacted] are on an overtime ban. This is unworkable and when a custody case or investigation demands overtime, officers are staying on to complete the job out of personal and professional pride for NO PAY!!

"I am not a miserable old dinosaur, I have [redacted] service and I love my job. Whistle-blowing on anything like this is career suicide within the [redacted] and I don't want to ruin my career.

"So I'd rather my identity not be published anywhere. I work in [redacted] and we are being told not to be proactive and investigate drug dealers because hay [sic] could cause overtime, but rather just do the work we are given."

Another email published by Mr Steele says "people who should be held (in) custody are being released on undertaking to avoid officers staying on to complete (a) case and incur overtime".

Mr Steele said the emails expose "the shameful reality of police cuts" and are "the tip of a very large iceberg".

He said: "First duty of government is to keep its citizens safe - but not from drug dealers it would appear."

The number of drug-related deaths has jumped by 15% in a year to the highest figure ever recorded, official figures released on Wednesday show.

Liberal Democrat justice spokesman Liam McArthur said: "These latest reports are hugely concerning on the same day that we have seen the largest ever recorded number of drug-related deaths.

"We need our police officers to be helping take drug dealers off our streets, not heading home early because we cannot afford to pay for overtime.

"This is just the latest in a series of deeply worrying reports from officers on the front line who say they are not getting the resources they need to do their jobs.

"Maintaining officer numbers is meaningless unless they are able to be proactive in their job and help keep streets safe.

"SNP ministers are letting officers down and failing communities across Scotland."