Vice Admiral Nick Hine, who is retiring, took the unusual step of posting a photograph of his exit letter on social media.

He wrote it had been an "honour and a privilege to serve" in the navy, but warned that change must happen.

Addressing "pedants and naysayers" in his letter, the departing Second Sea Lord wrote: "Please understand that the need for and, the value of change, is not a fantasy - if we don't transform we will fail and we will lose.

"It is that simple."

He added that his "bugbear" is the "great Navy has lost its ambition" and explained the desperation he has felt during his two and half years in the post.

"I have despaired at times that collectively we had become self-censoring, risk-averse and lacking in curiosity, confidence and critical thinking skills," Vice Admiral Hine wrote.

He blamed a "rose-tinted view of tradition" and a "spurious belief" that you can avoid risk by "sticking to a failing status quo" as reasons for not making changes and using these qualities.

"As Second Sea Lord I've challenged all of you to grasp the nettle, to be bolder in the face of the future and to truly be the people that you can be," Vice Admiral Hine continued.

"These last two and half years have been uncomfortable, uncertain and exciting in equal measure"

Throughout his time in the post, the senior officer unveiled the Navy's vision of what its fleet could look like in the future, including an enormous flying drone station in the stratosphere and an underwater flagship vessel.

"We have a mountain to climb to give the Navy a sustainable and successful future," he warned, adding that "significant progress" has already been made.

The outgoing Deputy Chief of Naval Staff, who served in tours of Northern Ireland, Kosovo and Iraq, praised those who "strive so hard for the success of the organisation", adding: "I can only pass on my genuine thanks.

But he added: "I am saddened that I have not been able to realise the level of ambition that I know is possible and needed."

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In his final remarks, the he called on the Royal Navy, Royal Marines and civil servants to be "ever braver and more audacious" going forward.

"Please pass on one piece of advice I received early in my career - strive to leave the service in a better place than you found it. Be better than yesterday," he said.

Mr Hine has now been replaced in his role by Vice Admiral Martin Connell.

Sky News has contacted the Ministry of Defence for comment.