South Africa's aviation authority on Friday stopped an Air Zimbabwe aircraft from taking off, sparking a furious protest from the airline.
The South African Civil Aviation Authority (SACAA) said the Boeing 767, registration Z-WPE, which was due to take off for Harare from OR Tambo International Airport at 1855 hrs, did not have a "foreign operators permit".
The decision effectively bans Air Zimbabwe from landing or taking off from South Africa, until the permit is obtained.
The grounded aircraft is the same that flew President Robert Mugabe to the 37th SADC Heads of State summit currently underway in Pretoria last Wednesday.
The latest drama comes as First Lady Grace Mugabe remains marooned in South Africa, with police minister Fikile Mbalula saying their borders have been put on "red alert" to stop her leaving after a criminal investigation was opened into claims she assaulted a model she found in a hotel room with her two sons.
Zimbabwe has sought diplomatic immunity for the First Lady, who is yet to comment on the allegations.
Air Zimbabwe passengers, who had checked in and were waiting to board, were not aware of the drama unfolding on the tarmac as flight UM462 taxied to the loading gate, before being surrounded by SACAA inspectors who wanted to carry out a safety oversight check.
The airline said in a statement released Friday night: "Our pilots parked at the gate and the plane was immediately surrounded by officials from SACAA who asked our pilots for their licences and documents for the aircraft.
"Finally, they asked for the foreign operators permit, which can only be issued by the South Africans and has no connection to the air worthiness of an aircraft. Our airline, as many others, does not have this particular document which has been a requirement for decades, but has never been issued to us."
The particular long haul B767 is said to be Air Zimbabwe's most reliable aircraft presently. Another B767 in its fleet is due for service while its B737 has been blacklisted for "noise pollution". An MA60 is currently used on the domestic routes.
An inspection report accompanying the Air Zimbabwe statement showed that the airline ticked all the boxes for the fitness of its aircraft, but SACAA inspectors had added, by pen, a new requirement for a foreign operators permit - which the airline was unable to produce.
All airports in the world require the same type of permit, which is issued by the civil aviation authority of that particular country. Air Zimbabwe officials say they have not been issued one for at least a decade, and the South Africans had never insisted on it until Friday - raising suspicions that the real target of their actions was Grace Mugabe.
Air Zimbabwe officials say they believe the decision to ban their aircraft from South Africa was made by overzealous junior staffers.
Said one senior executive: "We believe this decision to ground Air Zimbabwe, coming late on a Friday when offices that issue this particular permit are closed, is deliberate but one that has been taken at low level management.
"We'll engage the South Africans for a speedy resolution of this matter."
President Mugabe is not due back in Harare until at least after Sunday, when the two-day SADC Heads of State summit ends.
It remains unclear if the South Africans will release the aircraft for his flight home.
The same aircraft assumes the call sign UM1 when chartered by Mugabe, a technicality which could be used to get it released for repositioning at Waterkloof military airbase from where Mugabe is expected to take-off.
The actions by the South Africans could boomerang, with officials at the Civil Aviation Authority of Zimbabwe reportedly considering grounding South African Airways and South Africa Airlink aircraft in retaliation. SAA makes at least six flights to Zimbabwean destinations daily, carrying over a 1,000 passengers.
The grounding of South African aircraft would be an unprecedented escalation of the crisis, with potential to trigger a diplomatic incident without parallel in the modern history of the two neighbours.