By Charlotte Beugge

Shares in Tesco, Britain's largest supermarket group, slumped by 10% in early trading to 347p after the group admitted that its sales over the festive period were worse than expected.

In the six weeks to 7 January, Tesco sales were 2.3% down. According to the BBC, analysts were expecting a decline of 0.9%.

In comparison, rival Morrisons' Christmas trading figures were up 0.7% in the six weeks to 1 January and Sainsbury's were up 2.1% in the 14 weeks to 8 January.

The supermarket group, which claims in its adverts that "every little helps", said it expected profit growth for the current year to be "minimal" and that they will be "around the lower end of the current consensus range".

It remains the dominant UK supermarket with 30.1% of market share - but this has declined from 30.5%. However, Tesco's overseas sales were more robust and in the US were up 40.6%.

As one of Britain's largest companies, many millions of Britons will hold Tesco shares even if they don't realise it. If you have a tracker unit trust, you'll be invested in it and many managed investment and pension funds will have a stake in Tesco.

Many retailers have announced tough trading in the crucial Christmas period although some, such as the employee-owned John Lewis, announced healthy figures.

Retailing shares are cyclical stocks - which mean their performance is linked to economic factors - and the current age of austerity means we are spending less in the shops, impacting on their profits. A number of retailers haven't made it and have shut up shop.