The first female commander of a major Royal Navy warship has left her vessel after claims she had an affair with a shipmate.

Commander Sarah West, 42, took charge of Type 23 frigate HMS Portland in May 2012.

It is understood she has left the ship while the Navy carries out an investigation into the claims.

Her second-in-command has taken over the running of the vessel, but Cdr West is still the Commanding Officer.

The Navy inquiry will consider whether she breached the armed forces' code of social conduct, which governs personal relationships within the military.

Possible punishments for not meeting the high standards set out include a formal warning, reassignment and even termination of service.

A Royal Navy spokesman said: "We are aware of an allegation of a breach of the code of social conduct on board HMS Portland, which we are treating seriously.

"Anyone who is found to fall short of the Royal Navy's high standards can expect to face appropriate action. It would be inappropriate to comment further."

Cdr West made headlines when she became the first female commander of a frontline warship in the 500-year history of the service.

But earlier this year she explained how work commitments made it difficult to have a relationship.

She told the Daily Mirror: "I'm really proud to be the first woman but I'm not reinventing the wheel. Lots of women in the services have challenging roles. It's just that I happen to be newsworthy at the moment.

"There are drawbacks though. Years at sea probably explains why I'm single. But every person in the military makes sacrifices."

Cdr West, who was born and raised in Lincolnshire, studied maths at the University of Hertfordshire before joining Britannia Royal Naval College in September 1995.

When she was appointed to the role of Commanding Officer, she described it as the greatest achievement of her career.

She said: "'Taking command of HMS Portland is definitely the highlight of my 16 years in the Royal Navy so far.

"'It is a challenge that I am fully trained for and ready to undertake.

"'I am very proud to be taking command of a Type 23 frigate, which is an extremely flexible warship, and I'm looking forward to taking HMS Portland on operations with a professional and focused team."

HMS Portland sailed from Plymouth in January and is in the latter stages of a seven-month deployment which has featured Europe, Africa, South and North America.

She holds a crew of 185, has a top speed of 30.8 knots, weighs 4,900 tonnes and is 133m (436ft) long.

The stealth-designed ship has Harpoon and Sea Wolf missile systems, 4.5in guns and anti-submarine torpedoes.

According to a marine traffic website, the ship's last known position was the port of San Juan, Puerto Rico in the Caribbean on July 19.