Years ago, you needed a nursing qualification and be fluent in a foreign language in order to work for BA. But now, customer service experience is low and staff only stay for a short period of time.

BBC: BA Christmas strikes: Staff reaction

In other cases staff do not join existing schemes, typically because they are low paid and fear they cannot afford to contribute, or because they work in industries such as catering where there is a high turnover of staff who stay in jobs for only short periods of time.

BBC: Pensions auto-enrolment attracts cautious welcome