Wal-Mart, the US supermarket giant which owns Asda in the UK, faces a showdown with institutional shareholders at its annual meeting tomorrow over poor environmental and employment practices.

Some of the largest US pension funds, including the Illinois State Board of Investment, the City of New York Pension Fund and the General Board of Pension and Health Benefits of the United Methodist Church, have written to the company voicing concerns about its record on pollution, sex discrimination and illegal work contracts for immigrants.