South African Airways (SAA), Africa's largest airline, is not going to cancel its flights to Britain, but has warned of lengthy delays on return flights because a terror threat uncovered in London on Thursday.

Britain went to its highest state of terror alert on Thursday after the police arrested 21 people for a plot involving the hiding of explosive devices in hand baggage for detonating U.S.- bound flights.

"At this stage we are still planning on flying this evening," SAA spokeswoman Jacqui O'Sullivan said on Thursday.

"SAA's three daily flights to London landed this morning and passengers disembarked safely," she told the South African Press Association.

The airline was monitoring the situation "on a minute by minute basis, not even hourly," she said.

British Airways also said the flights leaving South Africa for London's Heathrow Airport would operate as normal on Thursday night.

However, it warned that onward flights -- particularly European services -- could be canceled or delayed because of severe airport congestion as a result of increased security arrangements.

Both airlines operate three flights a day from South Africa to London.

The South African Civil Aviation Authority convened a meeting on Thursday with "all people involved in airport security" to decide on a course of action.

Source: Xinhua