Image copyright PA Image caption Alan Stewart was originally due to join Tesco on 1 December

Troubled supermarket giant Tesco has said that its new chief financial officer, Alan Stewart, is joining the company immediately, more than two months earlier than originally planned.

Mr Stewart was previously in the same post at Marks and Spencer. He replaces Laurie McIlwee, who resigned in April.

On Monday, Tesco stunned the markets by saying it had overstated its half-year profit guidance by £250m.

Tesco's shares sank 11% on Monday and fell a further 4.2% on Tuesday.

The retailer has launched an investigation headed by Deloitte, and says it is now working to establish the impact of the issue on its full-year results.

It has also suspended four executives, including its UK managing director, Chris Bush.

Tesco is also believed to have suspended its UK finance director Carl Rogberg, its food commercial director John Scouler and the head of food sourcing, Matt Simister.

Appeal

Mr Stewart was originally due to join Tesco on 1 December, but his start date was brought forward after negotiations with Marks and Spencer.

BBC business editor Kamal Ahmed says he has been told that the move came after a direct appeal from Tesco chief executive Dave Lewis to his counterpart at Marks and Spencer, Marc Bolland, who "graciously" allowed Mr Stewart to leave early.

Mr Lewis's predecessor, Philip Clarke, stood down in July after his attempts to revive Tesco's fortunes through a £1bn turnaround plan failed.

Image copyright PA

Analysis, Kamal Ahmed, BBC business editor:

The announcement that Alan Stewart, the former finance director of Marks and Spencer, will start work today at Tesco - rather than the somewhat more leisurely 1 December - will bring some relief to investors, who have faced a battering over the last year.

A business facing an accounting crisis with no chief financial officer was not exactly comfortable.

Sir Richard Broadbent, the chairman, will hope that the move will quieten those who believe that he should consider his position.

He certainly spent yesterday, when the accounting crisis broke, desperately trying to move Mr Stewart's start date forward. In that, at least, he has been successful.

Last month, Tesco cut its full-year profit forecast from £2.8bn to £2.4bn, and said trading profit for the six months to 23 August was expected to be about £1.1bn.

As a result of the change to its profit guidance, Tesco has pushed back the release of its interim results to 23 October, from 1 October.

The investigation into the overstatement will be carried out by Deloitte together with Freshfields, the group's external legal advisers.

Discounters still winning

Tesco has been battling falling sales and a decline in its market share as discount chains such as Aldi and Lidl have gained in popularity.

And there is "no sign yet of recovery" in sales or market share for Tesco, according to latest figures from Kantar Worldpanel.

In the 12 weeks to 14 September, Tesco sales fell 4.5% from a year earlier and its market share remained at 28.8%, the Kantar report said.

The Kantar report also said annual growth in the UK grocery market over the 12 weeks was at a record low of 0.3%.

Asda was the only one of the "big four" - Tesco, Asda, Sainsbury's and Morrisons - to increase its market share, now at 17.4% and sales were up 0.8% on the same period last year.

The report confirmed that discount and premium supermarkets were still winning market share from bigger rivals.

"Aldi has continued its run of double-digit growth, which now stretches back to February 2011, by recording a sales increase of 29.1% compared with last year," said Fraser McKevitt, head of retail and consumer insight at Kantar Worldpanel.

"Similarly, Lidl has increased sales by 17.7%, showing that shoppers still have a strong appetite for the discount stores," he said.

Waitrose recorded a 4.5% rise in sales, which boosted its market share to 5.1%.