One of the largest shareholders in Darktrace is to sell a third of its shares in the British cybersecurity firm, in a move brokered at the end of a lock-up that prevents insiders selling their stakes in the recently floated company.

Deep Defence, a subsidiary of private equity firm Vitruvian Partners, is to sell 11m shares in Darktrace worth about £70m at the company’s closing price on Tuesday.

Shares in the Cambridge-based firm closed down a further 7% to 632.5p as investors concerned about a widespread sell-off of the company’s stock from Wednesday pushed its share price to a two-month low.

Vitruvian controls 31.84m shares in Darktrace, about 7% of its 153.25m publicly traded free float. It appointed brokers Berenberg to handle the sale to “eligible investors”. The exact price of the share sale was set to be determined through a rapid sale process called an “accelerated bookbuild”.

The move will raise further fears among investors of a significant sell-off among other top shareholders in Darktrace, which has seen about £2bn wiped off its market value in a little over a week.

The company’s biggest stockholders at the time of Darktrace’s flotation were Summit Partners, the Autonomy founder, Mike Lynch and his wife, Angela Bacares, and KKR and EBT.

Senior managers and directors at Darktrace hold about 5% of the company’s shares with Poppy Gustafsson, co-founder and chief executive, holding 2.77m shares, about 0.4%.

Last month, KKR, Summit and Balderton Capital sold £187.5m worth of stock, 25m shares when Darktrace was trading at 759.5p, a combined 3.6% stake. The 180-day lock-up agreement put in place at the time of Darktrace’s flotation in April was waived, allowing the sales to proceed. The lock-up, which applies to those investors who were shareholders at the time of the IPO, expires on Wednesday.

In August, the same trio of investors, along with Hoxton Ventures, sold 23m Darktrace shares at 620p valued at £143.5p.

Darktrace, which had been a star performer which joined the FTSE 100 last week, has seen its market value crumble from nearly £7bn to £4.7bn in a matter of weeks. The company is still trading at well over double the 250p at which it debuted on the London Stock Exchange six months ago.

Last Monday, two days before Darktrace replaced Morrisons in the FTSE 100 after the grocer’s takeover, an analyst note raising concerns over the business wiped a fifth of the company’s market capitalisation.