The oversight appears to have taken place at Delhi

Air India official

The airline is conducting a high-level inquiry into the security lapse which took place during an aviation security week in India designed to make flying safe.

A spokesman for Air India said the airline viewed the matter very seriously.

"There exists a case of lapse on the part of officials at the Delhi airport, following which these four have been put under suspension," an airline statement said.

Stowaways

It is thought two passengers bought tickets for a domestic flight, but then continued on to London without passports or valid tickets.

"The oversight appears to have taken place at Delhi, where the two should have disembarked but did not. There [was] a counting error by the staff," an Air India official was quoted as saying.

Air India says the passengers bought their ticket for the Bombay-Delhi leg of the journey with cash, giving false telephone contact numbers.

The airline says it was only when they collected the boarding cards in London that they found two extra passengers had been on board, though it could not confirm newspaper reports that the stowaways had hidden in the toilets.

Air India says it believes another couple on the same flight had bought tickets to Lagos via London but instead of going to the transit lounge they sought political asylum.