By Ian Walker 

LONDON--House builder Barratt Developments PLC will rejoin the FTSE 100 index after a six-year absence, and wealth-management firm St. James's Place PLC will join the top flight for the first time, FTSE International said Wednesday, replacing food ingredients company Tate & Lyle PLC and construction company AMEC PLC.

The changes take effect from close of business on March 21.

Barratt has seen its share price increase 23% since the start of the year as it, along with the other U.K. house builders, has benefited from the U.K. government's "Help to Buy" and "Funding for Lending" schemes. It was last in the top index between June and December 2007.

St. James's Place, which provides financial planning and wealth management services, helping individuals, trustees and organizations to achieve their financial goals, has seen its share price rise 14% since the start of the year.

By comparison Tate & Lyle's shares have fallen 22% as it has been hit by increasing competition in the artificial sweetener market and warned on Feb. 13 that profit for fiscal 2014 will be lower than its previous expectations. It said profit for the year ended March 31 will be in line with the previous year, when it posted a pretax profit of 309 million pounds ($515.08 million). It had forecast "another year of profitable growth" in November.

The FTSE 100 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.

While AMEC falls outside this range, it is the closest to the 110th threshold and would therefore move to the FTSE 250 to balance the top index, FTSE International said.

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

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