Why is the Barnett formula so important?

A political row has broken out after the leaders of the three biggest UK parties signed a pledge to keep the formula that decides changes in the Welsh government's budget.

David Cameron, Ed Miliband and Nick Clegg promised to keep what is known as the Barnett formula as part of the referendum campaign in Scotland. But critics say it short changes Wales by hundreds of millions of pounds a year.

BBC Wales parliamentary correspondent David Cornock explains why it is so important.