Freshfields Bruckhaus Deringer has been drafted in by Tesco to advise on a Serious Fraud Office (SFO) investigation into accounting irregularities at the supermarket giant.

Ali Sallaway, co-head of Freshfields' global investigations practice, is understood to be acting for the supermarket giant, alongside disputes partners Andrew Austin and Ian Taylor.

The trio were first instructed by Tesco last month, conducting an investigation along with accounting giant Deloitte after it emerged that the retailer had overstated its profit forecasts for the first half of 2014 by an estimated £250m.

The Financial Conduct Authority (FCA) had been investigating the accounting practices as a result, but will now turn over responsibility for the inquiry to the SFO, which announced the launch of a criminal probe this week.

The latest mandate cements Freshfields longstanding relationship with Tesco, with London retail sector group head Claire Willis acting as main relationship partner.

The magic circle firm plays a key role with the retailer on both M&A and regulatory investigations work. Last year Freshfields advised Tesco on a proposed joint venture with China Resources Enterprise (CRE) to give the supermarket chain a greater foothold in mainland China.

In 2012, the firm helped secure a partial victory in an appeal Tesco launched against dairy price fixing allegations made by the Office of Fair Trading, as well as helping set up Harris & Hoole, a new coffee business backed by Tesco.

Freshfields declined to comment on the SFO investigation.

In a statement Tesco said: "Tesco has been cooperating fully with the SFO and will continue to do so."