MI5 officer harassed fellow spy after 'passionate and volatile' relationship ended as they watched Andy Murray lose at Wimbledon, court told

Mark Barton denies harassment, sexual assault and common assault against female colleague

Secrets within these walls: the MI5 headquarters at Millbank by the Thames (Rex Features)

A MI5 officer sexually assaulted a fellow spy during a four-month campaign of alleged harassment and violence after their "passionate and volatile" relationship was ended as the pair watched Andy Murray lose at Wimbledon, a court heard today.

In a rare glimpse of the private lives of Security Service employees, a jury was told that the 28-year-old officer, known by the pseudonym of Mark Barton, had begun a romance in 2010 with a female colleague while they worked in the same section of the spying agency at its imposing headquarters on the banks of the Thames.

Southwark Crown Court heard testimony from Mr Barton's former lover, identified only by her MI5 personnel number of 2363, that at the start of the relationship the male agent had been "vivacious and dominating". But it fell apart after he had become prone to mood swings and outbursts which left her feeling "worthless".

Mr Barton, who denies four charges against him, is accused of resorting to what prosecutors said was a campaign of intimidation once the relationship broke down in the summer of 2011, and " behaved in a manner that was obsessive, intimidating, violent at times and frightening". His alleged actions included following and manhandling his former lover on public transport, visiting her mother unannounced and sending persistent emails, phone calls and texts.

At one point the agent is claimed to have forcibly kissed and sexually touched 2363 at her London flat and on another occasion threw her against the door of the large detached house after following her home from a Tube station.

The trial, which is being conducted in a screened courtroom to protect the identity of Mr Barton and other MI5 witnesses after a judge ruled that details such as their appearance and the precise nature of their work were "highly sensitive", was told by 2363 that their relationship was "intense and passionate, but it was also volatile".

In order to maintain the secrecy of the life of MI5 employees, jurors have been provided with maps showing locations including pubs close to its Thames House headquarters where the two officers went for drinks with colleagues. The pubs, along with other locations such as tube stations, can only be referred to in open court by letters of the alphabet.

The romance began to deteriorate from March 2011 after a holiday in which Mr Barton revealed he was still in contact with a former girlfriend. It concluded in a painfully public fashion while the pair were watching Murray lose in four sets against Rafa Nadal in the semi-final of Wimbledon on 1 July while sat in front of a large screen in Chelsea's Sloane Street.

Alison Morgan, prosecuting, told the court: "On this occasion the defendant's behaviour was particularly peculiar. He became angry as Murray began to play badly and he took that anger out on 2363, behaving in a way that she described as 'obnoxious'.

"For example, he suggested that she was lucky to be with him and that anybody would want to be with him... She walked away from the defendant and he shouted after her that if she did not come back the relationship would be over. She continued to walk away."

Giving testimony from behind a screen, the woman, said that Mr Barton had become angry after staff switched off the screen at 8pm while play was still ongoing then turned on her. She said: "I just started walking away. I said I had had enough. He said 'Do you know who you are and where you are?', meaning I was not good enough for him or to be in a nice part of London... I felt pretty worthless."

Despite the female agent's decision that the romance was over, it took another four months before the love affair turned completely sour, during which time they slept together on one more occasion and Mr Barton's behaviour allegedly became erratic, bombarding his former girlfriend with emails following a succession of rows and claimed public confrontations.

In one email, the agent, who described conducting "lonely trips" for his work and one occasion followed 2363 to work before trying to kiss her in a lift at Thames House, said: "As I said this morning (boring repetition, sorry) I'm desperately in love with you. I just can't turn that off or 'move on' however much I try. I have never felt this sad in my life... I know who the love of my life is even if she doesn't want to know - not ideal!"

In a reply to a separate email, in which Mr Barton revealed his father was suffering from cancer, 2363 replied: "I will be here as a friend for you, but I'm still really sorry in saying I'm not getting back into any form of relationship with you. We had an awesome time. I don't want to destroy those times with grief and anger... You don't have to play the big man your whole life. It's much more endearing if you are human."

The court heard that the woman approached her managers on two occasions about Mr Barton's behaviour towards her, which it is claimed was witnessed by colleagues, including her flatmate. Separately, Mr Barton drove to Northampton on an August bank holiday to introduce himself unannounced to 2363's mother and telling her during a two-hour conversation that he and her daughter "would be married within two years".

The court heard that matters came to a head after an alleged assault in December 2011 and police were called in by MI5. When interviewed, Mr Barton claimed 2363's allegations were "vexatious" and arose from her suspicion that he was still seeing his ex-girlfriend. The MI5 agent added: "I guess this is a woman scorned and [she] has decided to take retribution in the most painful way she knows... through costing me my job potentially."

Mr Barton denies charges of harassment, a single sexual assault and two counts of common assault between August and December 2011.

The case continues.