The co-founder of the clothing chain Superdry has cashed in on the group’s share price gains over the past year by selling a chunk of his holding, pocketing nearly £18m.

Julian Dunkerton sold 1m shares – 1.23% of the company, which was recently renamed from SuperGroup – at £17.80 a share, according to a stock exchange announcement.

Dunkerton, who founded the firm in 1985 with James Holder out of their Cult Clothing business and retains a hands-on role as Superdry’s product and brand director, remains the biggest shareholder, with a 25.36% stake. Nearly two years ago, he cashed in shares worth more than £53m to fund his divorce settlement.

The fall in sterling since the Brexit vote proved a boon for Superdry, boosting the fashion brand’s performance overseas, although sales growth has slowed recently. The brand has also benefited from its high-profile collaboration with the actor Idris Elba.

Superdry opened 50 new stores across 23 countries in the six months to the end of October, taking the total to more than 600. However, sales growth at its existing stores halved to 6.3%, with sales slowing in the run-up to Christmas.

Last September, Dunkerton and Holder launched a bonus scheme to share a fifth of their share price gains with the fashion brand’s 4,500 employees if the stock price hit an £18-per-share target.

It went through that level in October and hit an all-time high of £20.76 in early January. The shares, which floated on the stock market at £5 in 2010, were changing hands at £18.03 on Monday. They have gained 16% over the past 12 months.

If the shares rose by £5 above the £18 target, staff would share a £30m bonus. This would mean a £2,000 bonus for the company’s 2,600 full-time shop staff and junior head office employees. Store managers would collect between £28,000 and £75,000, while executive team members would get at least £300,000. Part-time employees will benefit, on a pro-rata basis. Board members are not included in the scheme.