According to acclaimed investor Leah Zell from Lizard Investors the shares of supermarket giant Woolworths Limited (ASX: WOW) could be set for an almighty tumble over the next two and a half years.

A report in the Australian Financial Review today reveals that Ms Zell has picked out Woolworths as a short idea, predicting that its share price could fall down to $14.51 by 2019. That’s a massive 38% lower than the current share price.

This would happen due to a drop in sales from $58.3 billion today down to $54 billion by 2019, together with a fall in its operating margin to just 3.5%.

Although I would be surprised to see its shares hit that level, the next few years will undoubtedly be tough for the company so it is possible.

The increasingly competitive environment means sales and margins are likely to come under significant pressure, especially with the emergence of ALDI and the potential arrival of fellow low-cost German supermarket retailer Lidl in Australia.

And then there’s US retail giant Amazon. Speculation is rife that Amazon is coming to Australian shores in the future and bringing its AmazonFresh grocery delivery service with it.

Although Bloomberg estimates that Amazon only has a one percent share of the US grocery market, it is the leader in the fast-growing US online grocery shopping market and is expected to have a 26 percent share in 2016.

Earlier this year Retail World estimated that online grocery shopping in Australia would double to be worth $5.8 billion a year by 2020.

If Amazon comes to Australia and has similar success then I feel it could spell trouble for the growth of Woolworths and rivals Wesfarmers Ltd (ASX: WES) and Metcash Limited (ASX: MTS).

Although Ms Zell makes a compelling case, I wouldn’t rush out to short Woolworths. Shorting is a risky strategy and one that could backfire if the Woolworths restructure is successful.

Instead I would suggest investors just play it safe and stay clear of the supermarket shares altogether at this point in time.

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Motley Fool contributor James Mickleboro has no position in any stocks mentioned. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.