Activist fund Sparta Capital is said to have built a stake in London-listed footwear company Dr. Martens, which has seen its stock plunge over the last year amid marketing and distribution issues in the U.S.

Sparta has been engaging with Dr. Martens about ways to improve the bootmaker’s financial and operating performance after amassing a position worth tens of millions of pounds, according to a Sky News report.

Dr. Martens shares were up 5% at 153.50 pence each as of 9:26 a.m. Monday in London trading, giving the iconic brand a market value of about $1.5 billion pounds. However, the stock is still down 19% year-to-date and a far cry from the 510 pence it was trading at in February 2021, shortly after its flotation.

Two months ago, Dr. Martens said that supply chain bottlenecks and lackluster demand for its shoes in the U.S. had eaten into its profits for the fiscal year ended March 31, and warned about more pain this year. The shares fell 11% on that news.

London-based Sparta was founded in 2021 by former Elliott Management executive Franck Tuil. Late last year, Sparta urged British engineering consultant John Wood to buy back stock to address its ‘‘material undervaluation’’ and avoid a takeover, a call the activist renewed this May.