LONDON (Reuters) - Shares in British sportswear retailer JD Sports Fashion underperformed a stock market rout on Monday, reflecting both its heavy exposure to key partner Nike and U.S. tariffs.

Ahead of a trading and strategy update scheduled for Wednesday, shares in JD Sports were down 6%, taking losses over the last month to over 18%.

The FTSE 100 index of blue chip stocks was down 4% on fallout from U.S. President Donald Trump's tariff announcement last week.

After last year's purchase of the Hibbett chain, just under 40% of JD Sports' global sales are made in the United States. Nike products account for about 45% of the group's sales.

Last week, Trump hiked duties on imports from a wide range of countries, including 54% on goods imported from China and 46% on goods from Vietnam.

JD Sports' exposure to U.S. tariffs is both direct and indirect.

Its direct exposure to U.S. tariffs is on its own label products that are sold in the U.S., much of which is sourced from China. Analysts at PanmureLiberum estimate this to be about 4% of group sales.

JD Sports also has indirect exposure, as the third party brands it sells in the U.S., including Nike, Adidas and On, are highly dependent on Vietnam and China.

While the brands have worked hard to reduce their U.S. sourcing exposure to China, they now have the problem that Vietnam is also under high tariffs.

"There is a good chance that the impact will be shared by the brands and the retailers like JD ... but it is definitely going to weigh on sector volumes," the PanmureLiberum analysts said.

JD Sports had already warned on profit in January, after weaker trading in Britain and the U.S. and promotional activity at competitors hurt sales.

Even before the tariffs, Nike warned last month of another quarter of sales decline.