Online grocery delivery service Ocado, which is pressing ahead with its IPO this week, is expected to float at about £800m, rather than the £1.1bn enterprise value its directors had been aiming for.

The company is likely to begin trading its shares in London on Wednesday, despite a barrage of criticism from analysts who say the business and its expansion plans have been overvalued.

Ocado, which has never reported pre-tax profits since its inception 10 years ago, last month confirmed plans to raise a total of £400m in shares, including an offer to customers who had this year spent more than £300 at its online store, which delivers Waitrose groceries.

However, reports say Ocado is believed to have raised only about £6-10m from customers, far short of the £50m it had set.

The company's shares - with their listing managed by Goldman Sachs and JP Morgan Cazenove - are expected to be set at the lower end of a 200-275p price band.

Ocado has responded to criticism citing a £100m capital expenditure facility at its disposal, but made no mention of the competitive threat posed by Waitrose delivering its own groceries.