Britain's newly-elected opposition leader, Jeremy Corbyn, suffered the first rebellion against his leadership on Wednesday in a parliamentary vote that exposed divisions over his economic strategy and policymaking process.

A group of Labour lawmakers defied strict instructions to vote against a plan put forward by the Conservative government to enshrine into law a commitment to get rid of the deficit and target a budget surplus.

The vote was set up by the Chancellor, George Osborne, as a way to pounce on disarray in the Labour Party since it elected Corbyn – a veteran lawmaker whose promised new consensual style of politics has exposed him to charges of a lack of direction.

The charter was approved by 320 votes to 258. An estimated 20 Labour MPs voiced disapproval of their own party's leadership by abstaining in the vote.

The rebellion is a blow to Corbyn, who despite saying he does not want to enforce decisions on others, is keen to stamp his authority on a party whose members are increasingly at odds with each other.

The protest followed a public spat over the competency of the new leadership after a policy U-turn by the Labour finance spokesman, shadow chancellor John McDonnell.

Earlier this week McDonnell said Labour was withdrawing its support for the government's so-called 'Charter for Budget Responsibility', two weeks after he pledged the party would back it.

"I suppose I should deal straightforwardly with the issue of the U-turn... Embarrassing? Yes of course it is," McDonnell said in parliament, appealing for support from his own lawmakers and drawing mocking laughter from Conservative opponents.

He said he had tried to "out-Osborne Osborne", referring to the finance minister's reputation as a fierce political tactician. But he dismissed the charter as a political weapon and said the austerity underpinning it inflicted misery on workers.

The government's pursuit of deficit reduction has been one of the Conservative Party's defining goals since it took power in a coalition in 2010. It then promised to all but eliminate the deficit by 2015. The government has so far only halved it.

Some economists have questioned the wisdom of Osborne's plan. In June, 77 economists, including leading academics from Britain and abroad, signed a letter which dismissed the fiscal charter as "nothing more than an attempt to outmanoeuvre his opponents" and having no basis in economics.

But with opinion polls suggesting the public does not see Corbyn as a leader, the Conservatives are keen to present Labour as incapable of running the economy – a perception that helped deal the party a crushing election defeat in May.

Osborne, seen by many as a likely successor to Prime Minister David Cameron, said Labour's vote against the charter showed a lack of fiscal credibility. "It's not a political gimmick to have sound public finances," he said.

"In my experience, shadow chancellors come and go but what is permanent is the economic approach the Labour Party is committing itself to tonight: they are becoming the permanent party of fiscal irresponsibility."