The machines are 24 years old and some people told the BBC they have fare dodged after coins have been rejected or the machines have not been working.

Director general of Nexus Mike Parker said the machines will be at the end of their working life in four years' time.

Then a new system could use hi-tech smart cards making travelling easier.

Smart cards

Some passengers have complained to the BBC that some ticket machines are too high for disabled access while others reject coins for no apparent reason.

Passengers have also complained at paying fares while fare dodgers travel without a ticket, facing just a small fine if caught.

Nexus runs regular drives to tackle the problem of fare dodgers.

Their names are now featuring on "name and shame" posters put up around the Metro system.

Mr Parker said improvements in the ticket system are planned.

He said: "The machines have probably got another four years' life in them and we are going to have to invest in their complete replacement then we will look at plastic and notes.

"Probably by then we will be moving to smart cards where a single card used for transport and other things."