Tesco boss says to show impact of accounting error next week-email

* Lewis says expects to give "clear and accurate" indication

* Tesco (Xetra: 852647 - news) reports delayed H1 results next week

LONDON, Oct 17 (Reuters) - The new boss of Tesco has told staff he expects to be able to give a "clear and accurate indication" of the impact of a 250 million pound ($402.18 million) accounting mistake when the grocer reports delayed first-half results next week.

The 95-year old supermarket is suffering its worst ever crisis after issuing three profit warnings in 64 days this year and discovering an accounting mistake that overvalued first-half profits by 250 million pounds.

Tesco is due to issue its first half earnings on Thursday and Chief Executive Dave Lewis, who has been in the role for just over six weeks, said the figures will give the market clarity over the firm's finances.

In a memo emailed to staff, a copy of which was obtained by Reuters, Lewis said: "By next Thursday, Alan Stewart (CFO) and I expect to be in a position to give the market a clear and accurate indication of our income for the first half of the year."

The firm is expected to update the market on the progress of its own investigation, being conducted by auditors Deloitte and law firm Freshfields, into the accountancy error.

A spokesman at Tesco declined to comment on the memo, in which Lewis thanked staff for their cooperation with the investiation, when contacted by Reuters. Tesco launched an investigation into its finances when it announced the black hole on Sept. 22 and it had not been clear how much of an update the market would receive next week.

Tesco has suffered over the last year with its big out-of-town stores losing favour as shoppers buy more locally and online, while discounters Aldi and Lidl and upmarket chains Waitrose and Marks & Spencer (Other OTC: MAKSF - news) squeeze the middle ground.

On Tuesday, industry data published by Kantar Worldpanel will show Tesco's sales and market share performance over the last three months.

In his memo, Lewis told staff that performance in its core business had improved.

"Our recent performance in Food has been one of our strongest for a very long time."

He also said that the firm will be adding thousands of additional hours to cope with anticipated extra demand over Christmas, traditionally the busiest time of the year for British grocers but which saw poor sales figures last year.

Tesco's interim results on Thursday will cover the six months to Aug. 23. (1 US dollar = 0.6216 British pound) (Reporting By Costas Pitas; editing by James Davey and Emelia Sithole-Matarise)