A judge has granted Donald Trump's demand for a "special master" to oversee the case into his handling of classified materials.

Mr Trump is being investigated for allegedly taking documents with him when he left the White House.

But the "special master" is an independent lawyer who decides if any of the records are covered by attorney-client or executive privilege.

The move is seen as a blow to prosecutors and a win for Mr Trump.

The former president had asked for the special master to be appointed, while prosecutors had strongly opposed it - and the appointment is likely to slow down the justice department's ongoing criminal investigation.

That is because the judge has banned the government from reviewing or using the seized materials for its investigation until the special master's independent review has been completed.

Department of Justice officials previously said that documents stored at former President Donald Trump's Florida home were likely to have been concealed as part of an effort to obstruct an FBI investigation.

The authorities say these documents should have been handed over to the National Archives - which US presidents are legally obliged to do upon leaving office.

Mr Trump, meanwhile, denies any kind of wrongdoing, arguing that as president he had declassified all the documents, and that they were kept securely at his Mar-a-Lago home.

The Department of Justice resisted Mr Trump's request saying that any presidential records seized in the FBI's 8 August search of Mar-A-Lago "belong to the United States, not to the former president".

It is not clear how long the special master's review will take.

Mr Trump's lawyers have argued some of the documents are covered by attorney-client privilege - a part of US law that allows people to keep conversations with their lawyers private.

His legal team have also claimed "executive privilege" over the documents, which allows presidents to keep certain communications under wraps.

However, some legal experts argue that these moves are merely a tactic to delay proceedings - since the documents have already been looked at.

Before the decision was handed down, law professor and US constitutional law expert Aziz Huq told the BBC the demand for a special master was simply "throwing sand in the gears".

"It's unclear why a special master is required. A special master is usually used before the government sees the underlying material, and here the government has seen them," the former law clerk for Supreme Court Justice Ruth Bader Ginsburg said.

Former federal prosecutor and former deputy attorney general for New York, E Danya Perry, said: "What really strikes me here is it's too late... you can't unscramble the egg or put the genie back in the bottle. These documents have been reviewed."