Police Scotland has stressed "there is no overtime ban", after a rank-and-file whistle-blower claimed officers have been told to abandon drug investigations in case they have to work additional hours.

The whistle-blower said he risked "career suicide" by revealing his concerns to the Scottish Police Federation (SPF).

READ MORE: Police Scotland officers 'told to drop drug dealer probes to avoid overtime'

Calum Steele, the SPF general secretary who published an email from the whistle-blower on Twitter, said the "sensational" claim is just "the tip of a very large iceberg".

The email states: "Officers... are on an overtime ban.

"Whistle-blowing on anything like this is career suicide... so I'd rather my identity not be published.

"We are being told not to be proactive and investigate drug dealers because (they) could cause overtime, but rather just do the work we are given."

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

READ MORE: Police Scotland officers 'told to drop drug dealer probes to avoid overtime'

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.

Police Scotland said there is no specific cap on overtime spending but that all costs and spending are being closely monitored.

Mr Steele said efforts to reduce overtime, which is easily identifiable in police figures, has increased costs in other less transparent budget lines.

He told the Press Association: "While this example is quite sensational it is far from the worst example of the cuts that have come in my direction.

"On the day that figures show that drug deaths are rising we have a police service which, as a consequence of lack of finance, is unable to pursue those that are peddling the misery of drugs.

"That is a pretty horrific state of affairs."

READ MORE: Police Scotland officers 'told to drop drug dealer probes to avoid overtime'

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.

Mr Steele added: "Overtime is a very effective use of police resources. It isn't affected by mileage, it doesn't necessitate accommodation, it doesn't impact on sickness, it doesn't result in abstractions elsewhere, and when issues progress to court it keeps the witness list small because you have a small number of officers dealing consistently with events throughout."

He said the Scottish Government must now "put its hand in its pocket" to protect the public.

"The service is not over budget - it is under-funded," he said.

"It's clear from the evidence I'm hearing, and other evidence in the public domain, that the severe lack of finance is hindering the police service."

READ MORE: Police Scotland officers 'told to drop drug dealer probes to avoid overtime'

A Police Scotland spokesman said: "There is no overtime ban in operation.

"Officers across Scotland continue to focus on meeting local priorities, including tackling drug dealers in communities, drug supply at local, national and international level and reducing the harm caused by drug abuse."

A Scottish Government spokeswoman said: "We will do everything to tackle the scourge of illegal drugs, by applying the full force of the law to hitting dealers who peddle misery that blights the lives of so many, while supporting those living with an addiction.

"We are committed to protecting the police revenue budget in real terms for the entirety of this Parliament, delivering an additional £100 million of investment over the next five years, in addition to £55 million of reform funding in 2016-17.

"Clearly, it is for SPA and Police Scotland to determine the best possible use of the budget according to national and local priorities."

READ MORE: Police Scotland officers 'told to drop drug dealer probes to avoid overtime'

Scottish Labour justice spokeswoman Claire Baker said: "The SNP cannot continue to live in denial over their failure to fund our police properly."

Scottish Liberal Democrat justice spokesman Liam McArthur said: "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."