By Ian Walker

LONDON--Wm. Morrison Supermarkets PLC (MRW.LN), the fourth largest U.K. grocer by market share, is expected to lose its place in the FTSE100 index later this month after more than 14 years, when the latest quarterly reshuffle is announced on Wednesday, based on indicative data from FTSE International Monday.

Wm Morrison, which joined the index in April 2001 is expected to be replaced by satellite firm Inmarsat PLC (ISAT.LN). Inmarsat was previously a member of the FTSE100 index between September 2008 and December 2011.

Shares of Wm Morrison have fallen 15% over the past 12 months and at 1350 GMT were trading 0.6 pence, or 0.35% lower at 170.4 pence. Supermarket retailers have been facing intense competition in recent years and losing market share to German retailers Aldi and Lidl as consumers search for cheaper prices. Morrison's market share in the 12 weeks ended April 26, 2015 was 10.9%

Last month the company reported a fall in first quarter sales and said it expects to incur one-off costs of 30 million pounds ($46.43 million) to GBP40 million during fiscal 2016 year-end as a result of restructuring costs following the arrival of its new chief executive officer. In the 13 weeks ended May 3 total sales, excluding fuel, were down 1.1% and 5.1% lower including fuel. For the previous quarter, total like-for-like sales were down 2.6% and 5.1% including fuel.

Inmarsat shares, by contrast are up 36% over the past 12 months. At 1250 GMT they were 3.5 pence, or 0.35%, higher at 996.5 pence.

The changes will be based on Tuesday's closing share prices and take effect from the close of business on June 19.

The FTSE100 is a share index of the 100 most highly-capitalized companies listed on the London Stock Exchange. Any company that falls to 111th and below is automatically ejected from the top-flight index, while any firm that rises to 90 or above is automatically promoted.

Write to Ian Walker at ian.walker@wsj.com