Cyber attacks on UK law firms climbed by nearly 20% between 2014-15 and 2015-16, according to new research that found that 73% of the UK top 100 were the target of attacks last year.

The prevalance of cyber attacks has now grown by more than 60% in two years, according to PwC's 25th annual Law Firms Survey, with the number of top 100 firms experiencing an attack rising from 45 in 2013-14 to 62 in 2014-15, and 73 during the most recent financial year.

The numbers were highest among larger firms, with almost 90% of firms in the top 25 facing attacks last year.

The most common security incidents faced by firms were email phishing attacks to try to gain access to client money, which 84% of firms that had been victims of attacks had suffered.

Patrick Hill, a partner at DAC Beachcroft and head of the firm's professional risk team, said: "There has been a huge rise in these types of attacks over the last couple of years."

He explained that in these attacks, "the hacker poses as a third party when emailing or getting in contact with a firm and specifies that purchase money should be sent to a different account".

Fifty-five percent of firms targeted by cyber attacks had been victims of attacks with viruses or other malware, while 16% of those targeted had faced significant attempts to break into their firm's network.

Hill said: "You can see a big reputational threat to law firms on the wrong end of these data breach incidents. If you are a major law firm, the ability to ensure your clients' data is kept confidential is absolutely key to your standing."

Firms also face an internal threat, with 41% of those that suffered security incidents victim to incidents caused by staff.

In March, security company Flashpoint released a report naming 48 leading law firms that had been targeted by cyber criminals looking to steal M&A information. The report named firms such as Allen & Overy, Hogan Lovells, Freshfields Bruckhaus Deringer, Kirkland & Ellis and Sullivan & Cromwell as potential targets. However, there was no evidence that any of the law firms targeted by the attacks had any information stolen.

The security information was contained within a wider report into the financial performance of the UK's largest law firms by PwC.

The report found that a combination of growing pressure from clients on fees, increasing associate salary costs and declining productivity is causing profit margins to fall at the UK's largest law firms.

The challenging market conditions are also resulting in decreasing revenues at a quarter of the UK top 100, the study found – up from around 18% last year. Firms are largely maintaining average equity partner profits through "tight control" of their equity, however.

David Snell, the head of PwC's law firms advisory group, said that while the sector was in "good shape" overall, many firms have been left with too many lawyers (the total number of lawyers at the UK top 50 increased 7.6% during the past 12 months) after overestimating client demand.

"As confidence returned to the sector last year, firms increased headcount in anticipation of continued improving market conditions," he said. "However, with the market turning out to be more challenging than expected and with increased competition from US firms and new entrants, spare capacity is now an issue for firms. "

This situation is likely to be "exacerbated" by market uncertainty following the Brexit vote, he added.

Snell said firms are now facing a period of "significant financial investment" in order to keep up with technological advancements, and predicts that the use of artificial intelligence will become "widespread" across the industry.