Former Hogan Lovells litigation partner Christopher Grierson has been sentenced to three years in prison for defrauding the firm of £1.3m in false travel expenses.

Grierson, who pleaded guilty in March this year after being charged by the City of London Police with four counts of false accounting, was today (30 May) handed sentences relating to four counts of fraud – one of 15 months, two of 36 months and one of 12 months, to run concurrently. He is expected to serve a total of 18 months in prison.

Today's sentencing at Southwark Crown Court had initially been expected to take place on 3 May, but it emerged today in court that this was delayed due to complications relating to triple bypass surgery Grierson underwent around the time that his offences were uncovered by Hogan Lovells in early 2011.

Grierson (pictured) was convicted in March this year on all four charges covering 57 fraudulent travel claims worth £1.27m over four years. He was was represented in court by QEB Hollis Whiteman's Mark Ellison QC.

Ellison's lengthy defence speech in court today referenced Grierson's long fight with depression, which was more recently diagnosed as a bipolar disorder. According to the defence, this caused periods of highs and lows during Grierson's successful career as a litigator with Hogan Lovells, which led to "bizarre behaviours" that were grossly out of character.

Best known for working on the high-profile BCCI litigation, Grierson was dismissed by Hogan Lovells on 11 May last year following an internal investigation led by the firm's finance team. The investigation uncovered evidence of more than £1m in irregular expenses claims made in just over four years running up to the end of December 2010.

Today's sentencing revealed details of how Grierson had become increasingly indebted, with his illness rendering him incapable of revealing the full extent of his financial difficulties to his wife, with whom he is currently in divorce proceedings.

The defence said the loss of the BCCI litigation in 2005 had worsened his depression, exaggerating his low self-esteem, sexual insecurity and a sense of inferiority, and driving him to turn to call girls as well as an affair with a woman in New York on whom he spent £630,000 paying rent on her apartment, as well as giving her more than £280,000 in cash and other expenses.

Hogan Lovells reported the case to the Solicitors Regulation Authority in May, with the regulator's investigation put on hold when the firm reported the case to the City of London Police in early June after Grierson repaid the money in full.

Today's sentencing also revealed that it was a "close associate" who stepped forward to pay back Hogan Lovells the money by the end of May 2011, and that Grierson has subsequently paid the friend back, with interest, with funds stemming from the sale of his house in France, by February this year.

The defence also called on several character witnesses, including senior QCs and Hogan Lovells itself, to prove that the fraudulent behaviour was out of character for Grierson.

Handing down sentencing, the court took into account that the crime was planned, that it was a severe breach of trust and that it involved a substantial sum of money but against this added that the mitigating factors – including mental illness, repayment of the stolen money, his heart condition and the fact he has been ruined financially and professionally – were impressive.

Based on precedent, the maximum jail term would be seven years' imprisonment but Grierson was handed the reduced sentence in part due to his doctor's advice that a lengthy prison sentence could increase an already significant suicide risk.

Detective Chief Inspector Perry Stokes of the City of London Police, who led the investigation, said: "Christopher Grierson's greed was his downfall. He abused his status as a well-respected partner within a high profile law firm to steal more than a million pounds from the company.

"This was a serious breach of trust and once he started offending, it was inevitable that he would come to our notice. The sentence passed today demonstrates how seriously the court views such cases."