The two companies have been targeted by Crispo (Citizens' Recovery of Indigenous People's Stolen Property Organisation) for what is called 'ethical shoplifting'.
Crispo claims that Brazilian mahogany sold by the two companies has been illegally imported, after being illegally logged in the Amazon. The group is being advised by Michael Schwarz, a solicitor at Bindman & Partners, who is helping them draw up a legal case against the retailers.
'A large quantity of mahogany in Brazil, especially in the Amazon in Indian reservations, is unlawfully logged, both under criminal and civil law, and exported unlawfully,' said Mr Schwarz.
'We have been looking into whether retailing of hardwood products in this country amounts to a criminal offence, and whether anyone can bring a civil case against them. The difficulty is proving any item in this country has come from an unlawful source. Even if it is unlawfully logged, there is the question whether it is unlawful to sell it in this country.'
Items removed from Jewson's and Latham's premises will be handed into police stations or left at the Attorney- General's office, with a request for legal action against the company. Retailers subjected to earlier mass shoplifting have decided not to prosecute - to avoid damaging publicity.
When Crispo handed in shoplifted mahogany products to the Attorney-General's office in June, it was supported by a group of MPs. They included Simon Hughes, the Liberal Democrat spokesman on the environment; Labour's Ken Livingstone; and Cynog Dafis, the Plaid Cymru MP.
Retailers can expect not to be targeted if they have signed the World Wide Fund for Nature's 1995 Target, which aims to eradicate the sale of hardwoods not grown sustainably. Signatories include Do It All, MFI, and B&Q.
But Crispo is a loose organisation, and local groups act independently. As a result, mahogany was removed from B&Q's The Depot store in Leicester in June this year by local Crispo supporters, contrary to the campaign's national policy.
The Leicester group said that morally the company should not be selling old stock but should give the products back to Brazilian Indians - or at least give them the proceeds of any sale.
The Timber Trade Federation, whose members include Jewson and Latham, argues that the WWF's 1995 Target is unnecessary, as its members commit themselves to an accord with Aimex, which represents loggers of Brazil's Para region, origin of most of Britain's sawn mahogany.
'The purpose of the accord is that products are sourced in accordance with Brazilian and UK law,' said a spokesman. 'Aimex is a legitimate Brazilian organisation.'
WWF says that it welcomes the Aimex agreement, but is not convinced that it is effective. 'If 80 per cent is extracted illegally, and if all the major suppliers sign to say they only extract legally, then it doesn't add up, it isn't credible,' says Francis Sullivan, the WWF campaigner on forests.
'In 10 to 15 years there will be no Brazilian mahogany left. That would be a social and ecological disaster, with thousands of jobs lost in Brazil, and hundreds in Britain.'
Amanda Burton, Jewson's company secretary, said: 'We stress that our wood must come from legally logged sources. If Crispo has any information to the contrary we would like to be told. We would be very upset if it were true.' Latham declined to comment.