Teams from the Brazilian air force, police and volunteers will scour an area of jungle and sea in the north-east of the country.

As the operation entered its fourth day, sources said efforts were being made to obtain extra resources in a bid to find the aircraft.

The search was launched on Friday night when the twin-engine Cessna 310 aircraft lost contact with air-traffic controllers minutes before it was due to land in the coastal city of Ilheus.

Nigel Hodges, 52; Alan Kempson, 46; Sean Woodhall, 43; and Ricky Every, 41, were travelling from Salvador to Ilheus as part of a trip to check out a proposed housing development planned by Worldwide Destinations.

The international marketing and development company, which has its headquarters in Malaga, Spain and a UK office in St Neots, Cambridgeshire, says it specialises in creating luxury property developments in new emerging markets.

Entrepreneur Mr Woodhall, a Swiss resident, is its president, according to Michael Lynn, a long-time friend and colleague.

Mr Lynn said: "Everybody is hoping that there will be a positive outcome. We have to stay positive, but it has been a long time now.

Mr Every lives in Brazil and works for Worldwide Destinations there, Mr Lynn said.

He added that Mr Kempson and Mr Hodges - who are directors of a British sub-prime mortgage business which currently has financial difficulties - were potentially going to do some work with Worldwide Destinations.