President Bingu wa Mutharika of Malawi has died, doctors and cabinet ministers have told the BBC, but the lack of a formal announcement is leading to widespread anxiety.Mr Mutharika, 78, suffered a cardiac arrest on Thursday and state media say he was flown to South Africa for treatment.There are fears that his death could lead to a power struggle. Both the UK and US have called for the constitution to be respectedAccording to the constitution, the vice-president takes over if the head of state is incapacitated or dies in office.But Vice-President Joyce Banda and Mr Mutharika fell out after a row over the succession in 2010, and she was expelled from the ruling Democratic People’s Party (DPP). Mr Mutharika’s brother, Foreign Minister Peter Mutharika, was chosen instead of Ms Banda to be the DPP’s presidential candidate in the 2014 elections.According to the Malawi constitution, the person who is in charge right now - the only person who can make lawful and binding conditions - is the vice-president. Any person who assumes the powers of the president will be committing treason. The Malawi Law Society will commit to make sure the vice-president takes up the position of the president. If it means going to court to get an order restraining an attempt by other people, we are ready to do that.He has been standing in for the president when needed during official occasions. Ms Banda recently told the BBC she had not spoken to Bingu wa Mutharika for more than a year.On Friday evening, Ms Banda told a news conference that she was waiting for word from South Africa about the president’s condition.The BBC’s Raphael Tenthani in the main city, Blantyre, says ministers have been locked in non-stop talks about the situation.The doctors and ministers say that Mr Mutharika’s body was taken to South Africa while a decision is reached about what to do next. Government sources have told the BBC that efforts to resuscitate President Mutharika after his cardiac arrest had failed and that he was "clinically dead” on Thursday.‘The laws are clear’ UK International Development Secretary Andrew Mitchell is among international leaders calling for a transition in line with the constitution. Johnnie Carson, Assistant Secretary for African Affairs at the State Department, said the US is "concerned about the delay in the transfer of power”. "We trust that the vice-president who is next in line will be sworn in shortly,” he said in a statement. Malawi Law Society president John Makhwawa told the BBC’s Focus on Africa programme that people were very anxious because of the lack of official information.He said that the vice-president should already have assumed powers and that anyone who attempted to govern in her place would be committing treason.Former President Bakili Muluzi - a bitter rival to Mr Mutharika - has also called on the authorities to make an announcement."It’s important that the government announces the condition of the president as soon as possible so that the nation is informed,” he told journalists.Mr Mutharika, a former World Bank economist, came to power in a 2004 election, after being backed by outgoing President Muluzi. Soon afterwards, Mr Mutharika left his United Democratic Front (UDF) to form the DPP, after accusing Mr Muluzi and other UDF leaders of opposing his campaign against corruption.Since being re-elected with a large majority in 2009, critics allege he has demonstrated an increasingly authoritarian streak.The president has been under mounting pressure to resign, amid accusations of nepotism and economic mismanagement.The criticism has led to a souring in relations with major foreign aid donors, especially the United Kingdom.Last year, Mr Mutharika expelled the UK High Commissioner, Fergus Cochrane-Dyet, after a leaked embassy cable quoted the diplomat as saying that the president could not tolerate criticism.The Malawian leader said he could not accept "insults” just because the UK was his country’s largest aid donor.In response, the UK expelled the Malawian envoy to London and cut direct aid.Malawi is one of the poorest countries in the world, with an estimated 75% of the population living on less than $1 (60p) a day.The country has suffered shortages of fuel and foreign currency since the UK and other donors cancelled aid.