Lucrative black cab licences are so scarce in Oxford they are changing hands for up to £70,000 a plate.

But cabbies fear a city council plan to abolish the limit on taxi numbers would leave them bankrupt. Meanwhile, drivers in the north of the county are mulling over the prospect of strike action about similar council-led plans to alter the number of licences.

Oxford has just 107 Hackney Carriage 'plates' - with a rare lottery held every four years, when a small number of plates go up for grabs. For many cabbies the licence plate is their pension or nest egg, which they plan to cash in when they hang up their keys.

But an Oxford City Council task force is thinking about lifting the cap on the number of licences - a move that would make the current plates worthless. Alan Woodward, secretary of the City of Oxford Licensed Taxis Association, said cabbies were fearful about the possible change.

He said the drop in plate value and a lack of customers would lead to financial ruin if the market became a free for all.

He said: "At the moment there are lots of drivers earning less than the minimum wage, as it does not apply to self-employed people. Lots of blokes are working 10 or 12-hour shifts, earning £40, because business isn't there. It's there on Friday and Saturday night, but you don't make a living on two shifts."

Mr Woodward estimated the going rate for an Oxford plate at £50,000 - but added he would not pay that price.

However he said drivers also faced spiralling insurance and fuel costs, and so relied on the sell-on value of the plate as a return on their investment.

He said: "Drivers can see everything they have worked for going down the pan," he said.

At Oxford Railway Station, cabbies were equally worried by the prospect of de-restriction the number of Hackney Carriages.

One driver, who asked to remain anonymous, admitted paying £70,000 for his plate. He said: "I would hope to get that sum back, but if it's derestricted, I won't. The main issue is there's not enough fares to go around."

Cabbie Philip Willett said: "Sometimes you are down here for an hour and 25 minutes waiting for a job. More taxis means less work and we struggle to make enough to live on at the moment."

City councillor Sid Phelps, who is heading the group investigating derestriction, said the group had been told £50,000 was the rate for an Oxford plate.

Mr Phelps added: "I am not happy they are changing hands for so much money. It is a bit like, if you're in the club, great; if not, then it is really difficult to get in."

A report drawn up by the group said de-restriction would increase taxi numbers benefiting passengers and reducing waiting times.