The mayor wants more children to use the Tube

They can travel on weekdays after 0930 GMT and all day at weekends and bank holidays, if they are with an adult.

Bus fees for under-16s have already been waived - the mayor wants to make the Underground more family-friendly.

But his Conservative rivals said the scheme was confusing and argued everyone should be paying lower fares.

"Any reduction in fares is good news," said Roger Evans, the Conservative transport spokesman on the London Assembly.

Free tube travel for under-11s will help make visiting London's many attractions more affordable Ken Livingstone

"But we now have a myriad of reductions and freebies on the London Transport Network and it can sometimes be difficult to keep track of which is which.

"It would be much simpler if Ken Livingstone cut out these bureaucratic schemes and just offered low fares for everyone."

Introducing free Tube travel will cost about �1m a year, including lost fares, but the mayor hopes to reap long-term benefits.

He said free bus travel had made a "big difference" to hundreds of thousands of London families.

"Extending it to under-11s on the Tube and the DLR (Docklands Light Railway) is the logical next step.

"Free Tube travel for under-11s will help make visiting London's many attractions more affordable, widen the horizons of young Londoners and encourage the next generation to use public transport as much as possible."