NICK Xenophon’s fledgling SA Best party has been rocked by the snap resignation of its only sitting member of Parliament, just seven months from the state election.

Upper House MP John Darley declared late Thursday he had quit the SA Best and Nick Xenophon Team parties.

He said he would remain in Parliament as an independent until his term finishes in 2022, prompting Senator Xenophon to demand that he quit parliament.

Mr Darley said he had argued with Senator Xenophon over the name of the new state-based SA Best party and policies on the proposed bank tax and electoral reform.

He said a key problem was that Senator Xenophon “just wants everything his own way”.

In the lead-up to the March state election, polling has shown Senator Xenophon’s party would garner about one in five votes, putting him in a prime position to contest marginal seats and play kingmaker if a hung Parliament results again.

Senator Xenophon rejected suggestions that Mr Darley’s resignation would damage the party’s chances of claiming at the 2018 poll. Instead, he said it provided a “clean slate” and he would reveal candidates next month.

“You’ll be seeing a lot more of me campaigning in South Australia,” he said.

Senator Xenophon said Mr Darley had jumped ship ahead of a meeting which had been scheduled for Friday morning, where the party planned to expel him for voting against party policy on electoral reforms in State Parliament.

He called on Mr Darley to quit Parliament to make way for NXT running mate Connie Bonaros.

“A lot of South Australians voted for him because of my endorsement and he ran as the independent Nick Xenophon Team candidate,” he said.

“I worked very hard to get him elected. Now that he’s no longer a team player, he needs to step down from the Parliament.”

It is understood disagreement had been brewing between the pair for months.

When asked about the relationship between them, Senator Xenophon said he was “not going to talk about other issues”.

“At a personal level I thought I was getting on quite well with him,” he said.

Mr Darley replaced Senator Xenophon when he entered Parliament in 2007.

It is not the first time Senator Xenophon has lost an ally following a falling out.

In 2007, Ann Bressington quit a parliamentary partnership with him and went on to harshly criticise him publicly.

Mr Darley said he did not give Senator Xenophon advance warning of his decision to announce his resignation.

He recently pledged to vote with Australian Conservatives MPs in the Upper House to block the State Government’s proposed contentious bank tax — and confirmed he would stand by that position.

There had been suggestions this had caused problems within NXT, but Senator Xenophon said that “is not true”.

“On the night (the tax) was announced, I said ‘Let’s keep an open mind’ but by the next day, after discussing it with John ... I came out and said I had serious concerns about this tax and its impact,” he said.

Asked to comment on the resignation, Premier Jay Weatherill said: “This is really for Nick Xenophon to explain.”

Mr Darley issued a brief statement on Thursday afternoon, saying his resignation was “effective immediately”.

“There are many things I could say as to why I have resigned from the parties however it is not my place to speak publicly about internal party matters” Mr Darley’s statement said.

“In my 10 years in Parliament I have done my best for South Australia.

“This move will allow me to continue this and I wish them well for their future”.

Responding to the announcement, Mr Xenophon, who represents SA in the federal Senate, tweeted: “John Darley MLC’s resignation from NXT and SA-BEST has averted his expulsion tomorrow due to breaches of party rules. I wish him well.”

In a tweet to The Advertiser, Mr Darley said: “Meeting was to DISCUSS this. Haven’t been able to discuss anything with him (Mr Xenophon) as he wouldn’t return my calls.”

John Darley MLC's resignation from NXT and SA-BEST has averted his expulsion tomorrow due to breaches of party rules. I wish him well. — Nick Xenophon (@Nick_Xenophon) August 17, 2017

Australian Conservatives MP Robert Brokenshire said he was not surprised to hear of Mr Darley’s resignation, but believed he would maintain his position opposing the bank tax.

“Mr Darley is a man of his word and I would expect him to stick with us in that vote,” Mr Brokenshire said.

“It’s no surprise to me at all (that Mr Darley has resigned from the party).

“The problem with the Nick Xenophon Team that people have been fooled by is that it’s not a team, it’s controlled by Mr Xenophon and it’s all about populist policy.

“Mr Darley would have found it difficult to accept that.”