Grant Thornton is set to become a FTSE 100 auditor as its client retailer Sports Direct International is poised to enter the top tier of the UK's biggest companies in the latest three monthly reshuffle of the index.

In its most recent trading update, Sports Direct reported sales up 18.2% to £613m in the 13 weeks to July 28, with gross profits up 23pc to £260m. Shares have been hovering at round 729p, giving the company a market value of £4.4.bn.

In this month's London Stock Exchange quarterly review of the index, Sports Direct will be joining the FTSE 100 from 23 September, when the new listings take effect, along with Coca-Cola HBC AG, and Mondi. In the rebalance, Wood Group (John), Eurasian Natural Resources Corporation and Serco Group, will leave the UK's leading index and enter the FTSE 250 Index.

GT has audited Sports Direct since the company was listed in February 2007, and the retailer was previously the largest of the five FTSE 350 companies which the Competition Commission noted as being audited by GT.

In its latest company report, Sports Direct said the audit committee had recommended the reappointment of GT for the 2014 financial year. In explaining its decision, the committee said it 'believes their independence, the objectivity of the external audit and the effectiveness of the audit process is safeguarded and remains strong. This is displayed through their robust internal processes, their continuing challenge, their focused reporting and their discussions with both management and the Audit Committee.'

The report shows that for the financial year ending 28 April 2013, GT was paid £83,000 for audit services, plus £585,000 for audit of subsidiary companies' financial statements, £260,000 for other services relating to taxation, and a further £86,000 for 'all other services'.