Labour MPs are already rebelling against its radical new leader Jeremy Corbyn

Jeremy Corbyn was elected on a sweeping mandate from Labour members last month to lead the British political party - but he's having a tough time getting MPs on board.

UK Chancellor George Osborne's new law requiring future government's to run a budget surplus when the economy is growing - in other words only spend slightly less than it raises in tax - passed in Parliament on Wednesday night.

21 Labour MPs defied the whip and abstained the vote. That means they went against direct party orders to reject the law.

It follows a call from Osborne for Labour MPs to rebel against their party and a surprise u-turn from Labour's shadow chancellor on the bill itself.

Labour shadow chancellor John McDonnell initially supported Chancellor George Osborne's new budget law. But on Tuesday he said the party would oppose it. McDonnell said yesterday he was embarrassed by how he had handled the whole thing.

Clearly this is a blow for Corbyn's authority as leader - the Chancellor of the party Labour is supposed to oppose managed to incite a rebellion within its own ranks. That's not good.

Perhaps the most embarrassing thing is that the rebellion didn't make a difference. The fiscal charter passed by 320 votes to 258, meaning even if those 21 MPs had opposed the bill it would have passed.