Uber, the the app-based taxi hailing service, has become wildly popular - with consumers if not black cab drivers - since launching in London in July 2012.
It's famously cheap. But this week's Tube strike - the whole network is set to be out of action for all of Thursday - might mean Uber cabs are most expensive than usual because of something called 'surge pricing'.
Under this system, fares cost 1.5 to 2.5 times the usual price at busy times, when demand is high.
Uber says this is to encourage more of its thousands of drivers on to the road. Supply and demand - when demand is outstripping supply, prices goes up.
"Prices increase to encourage more drivers to become available", as Uber says on its website. The company insists this is to "ensure reliability", that is, to make sure a car turns up in good time even at busy times.
During a Tube strike last year, Uber offered a 50 per cent discount for people sharing cars. The company reported a four-fold increase in demand compared to a normal weekday.
The company said last year it expected to have 42,000 drivers in the capital by March 2016, and that it had 500,000 users in London.
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