LONDON (Reuters) - Aldi, Britain's fastest growing grocer, more than doubled profit in its UK business in 2012, as it attracted 1 million more shoppers and got existing customers to spend more.

According to market researcher Kantar Worldpanel, Aldi ended 2012 with a market share of 3.2 percent. That has since grown to 3.7 percent in the most recent industry data as it and fellow discounter Lidl win share from the major supermarkets, including market leader Tesco.

The UK arm of the Germany-based Aldi empire said on Monday it made a pretax profit of 157.9 million pounds in the year to December 31 2012, up from 70.5 million pounds in 2011, an increase of 124 percent, as turnover jumped 40.6 percent to 3.89 billion pounds.

Aldi's sales growth partly reflects the opening of 34 new stores in 2012, at an investment cost of 116.5 million pounds.

It plans to invest a further 185 million pounds opening 50 shops in 2013. Its 500th store is due to open next month.

"We have a simple low pricing offer that customers really understand and we don't try to confuse them with the likes of multibuy promotions," said joint group managing director Matthew Barnes.

The firm also attributed its growth in 2012 to a focus on local, British sourcing. Its fresh meat sales have risen by more than 60 percent year-on-year for the last three years, while fruit and vegetable sales increased by over 49 percent in 2012.

Aldi is also attracting a broader demographic of shopper. According to market researcher Nielsen, almost one in three households now visits Aldi at least once a month.

This Christmas products available will include a luxury Serrano ham leg and a fresh three bird roast.

However, along with several other grocers Aldi was implicated earlier this year in the discovery across Europe of horsemeat products labelled as beef.

Tesco and No. 3 Sainsbury will both update the market on Wednesday.

(Reporting by James Davey; editing by Sarah Young)