Italian pesto sold by UK supermarkets contains substitute and often cheaper ingredients such as sunflower oil, carrot and even bamboo fibres, an investigation has found.
A test of 12 own-brand standard and premium pestos - all made in Italy - found a range of additions to the sauce's traditional four key ingredients of basil, pine nuts, parmesan and olive oil.
Which? found all of the standard pestos were made with between 42% and 49% basil, but also contained cheaper alternatives to the traditional ingredients such as cashew nuts instead of pine nuts, or a mix of both.
Olive oil was also substituted with sunflower oil, and parmesan with less expensive Grana Padano and Pecorino Romano cheeses.
All of the standard pestos, apart from Waitrose's £1.35 version, used thickeners such as potato flakes, nut flour, vegetable or even bamboo fibres.
Standard pesto from Sainsbury's and Tesco, both £1, and the Co-op (£1.19) also contained sugar, and the Co-op, Morrisons (£1) and Tesco listed water as an ingredient.
Marks and Spencer's standard jar, the most expensive at £2.10, contained carrot fibres.
All of the supermarkets' premium pestos were labelled 'alla Genovese', relating to the sauce's origins in Genoa, all contained between 38% and 50% basil as well as extra-virgin olive oil, Parmigiano Reggiano (parmesan) and pine nuts.
However, even these contained some surprising extras, with Morrisons (£1.95), Sainsbury's (£1.50) and Tesco (£2) all adding vegetable or bamboo fibres as thickeners and sugar, and Morrisons also listed water as an ingredient.
The products with the most authentic and traditional ingredients were Asda Extra Special Genovese Basil Pesto (£1.39) and Waitrose 1 Pesto alla Genovese (£2.70).
The watchdog said its investigation showed that a higher price did not necessarily mean that the product would be free of cheaper and possibly unwanted ingredients and urged shoppers to check the labelling.
Which? director of research Nikki Stopford said: "Pesto has become a staple food in the UK but our research shows that many shoppers may be getting more than they bargain for when it comes to the pesto they are buying - and not necessarily in a good way.
"Our advice is not to assume that all pesto contains the same traditional ingredients. Check the ingredients list if authenticity is important to you, or if you are trying to avoid certain contents, such as added sugar."
An M&S spokesman said: "The carrot fibre in our pesto ensures that the oil and basil doesn't separate, naturally improving its shelf life and helping it last longer for customers."