It comes as it was revealed that posties are suffering up to 4,000 attacks every year.
The attacks often result in severe injuries and “considerable trauma” and highlights the case for legislation to tackle dangerous dogs, said campaigners.
The independent inquiry was announced by Royal Mail chairman Donald Brydon, with the aim of making specific recommendations to reduce the number of dog attacks.
Mr Brydon said: “It is an offence to decency that good people should suffer these attacks when carrying out their daily jobs and serving the public.”
Dave Joyce, national safety officer of the Communication Workers Union, which has been campaigning for years for measures to tackle dangerous dogs, said: “This is a very positive and welcome development at a key moment and will definitely raise the profile of the issue and influence the public and political debate and strengthen the case for change which is overwhelming and must not be further delayed.”
The inquiry will be led by former high court judge Sir Gordon Langley and will study the number of attacks, which are running at between 3,500 and 4,000 a year.
More than 24,000 postmen and women have been attacked by dogs on their delivery rounds since 2006, leading the Royal Mail to spend over £100,000 on campaigns and equipment to try to reduce the risk of injury, including a device to put letters through doors to protect fingers.
Almost 400 postmen and women have taken time off sick in the past year after being attacked by a dog, while 4,100 working days have been lost, costing Royal Mail around £400,000.
The CWU said it believed up to 6,000 postal workers were attacked by dogs every year while on delivery rounds and called on the Government to strengthen the law as a matter of urgency.
The union said it received an assurance from Prime Minister David Cameron almost two years ago that he supported its campaign for tougher laws against dangerous dogs, but changes have not been made.
The CWU said recent incidents included extensive bruising and a deep flesh wound suffered by Devon postwoman Patricia Prew inflicted on her by a golden retriever, which attacked her as she entered the front garden of a house in the coastal town of Budleigh Salterton.
“It just grabbed hold of me and pulled me down and then it bit me so deeply that it took a chunk of my muscle from that part of my arm,” said Ms Prew, who was off work for six weeks.
In August 2011, Andrew Berge suffered severe damage to his left-hand ring finger and tendon when a dog bit him at an address in south west London.
“As I posted an item through the letterbox, a dog bit and held on for four to five minutes,” he said.