Coin-operated machines to book minicabs on the streets will be installed in London's financial district next month to ease the cab shortage.

The trial scheme by the Corporation of London will introduce the temporary boxes from 6.30pm to 5am to those areas where people are most likely to get stranded.

Trying to hail a black cab in the square mile has become increasingly difficult over the past few years.

The shortage has been made worse because City firms block-book cabs to take their employees home after work.

Critics believe the scheme will be a magnet for touts

A spokesman for the Corporation of London said: "You will see black cabs lined up outside the big firms and lots of cabs in the city, but none you can hail in the street, which is very frustrating."

As only black cabs can be legally hailed from the street, those out late have had to turn to minicab touts to get them home.

Passengers will be able to put �1 into the new machines which locate and send out the nearest licensed minicab, if one is available, giving the passenger its registration number.

A similar scheme has been running in Australia and was introduced to the Corporation of London by licensed minicab company Capital City Chauffeurs.

Tout magnet

But John Thomas of the Licensed Taxi Drivers' Association, which represents black cab drivers told BBC London: "It'll be a magnet for touts. Anybody standing there is going to have the life plagued out of them.

"I can see arguments breaking out - if that's what the City fathers want well good luck to them - we don't want it."

Police say they will know where to find the culprits.

Detective Inspector Bernie Emslie, from the City of London Police, said:"There's a risk but it's a two-way street because while taxi touts know there will be vulnerable people there, so will we so it will be easily policed."

"It isn't the answer, but it's an answer.

"There's an increasing amount of assaults by these people (unlicensed minicab drivers) so we have to do something about it."

He hopes a campaign in October to raise awareness of the dangers of using unlicensed minicabs will have the same effect as that against drink driving.

The six-month trial could be rolled out across London if successful.