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Shares in online grocer Ocado have jumped by a third after a jump in sales during 2011.

The firm, which mostly sells Waitrose goods, said that customer orders in the four weeks to Christmas jumped 22% from the previous year, to 405,700.

"We remain optimistic about continued sales growth," said chief financial officer Andrew Bracey.

Overall, sales rose 17% to £643m in the year to the end of November from the previous year.

Ocado's own-brand sales were up 88% in the four trading weeks to 25 December.

The average order size for the final quarter of the year was £109.40, as compared to £112.10 in the equivalent period in 2010.

Last year, Waitrose announced it would be expanding its online presence to within the M25 area, potentially threatening Ocado's own business.

Ocado was founded in 2001 by three former Goldman Sachs bankers.

Its shares floated at 180 pence in July 2010, but have since fallen to about 65p currently.