Ratings firm issues Tesco warning
The embattled grocer has endured one of the most challenging periods in its history after it issued its first profit warning in 20 years.
The agency also warned that chief executive Philip Clarke's £1 billion plan to invest in improving customer service in its UK stores will negatively affect its trading margins.
A downgrade would make it more costly for the retailer to fund any expansion or turnaround plans in the future and make it harder for the supermarket to keep prices low for customers.
A statement from S&P said: "Tesco's operating performance will likely continue to be dampened by sluggish household spending in the UK as a result of nominal wage growth, a fragile labour and housing market, and high household debt burden."
Investors held their nerve in the wake of the warning as shares were broadly flat at 318p.
The stock has plunged 23% since the profit warning at the start of the year, as its market share has been chipped away by cheaper rivals such as Aldi and Lidl.
Tesco admitted that its £500 million Big Price Drop launched last year failed to impress customers but has revamped the initiative to focus more on giving customers special offers and money-off coupons.
Mr Clarke also unveiled plans to spend £200 million on extra staff and improving levels of service after a trial in 200 stores delivered a 1.1% sales boost.