A key decision about attendance at the Geneva 2 talks had also not been made, with a senior SNC official saying that more details were needed.

“We are waiting to hear the outcome of the Kerry-Lavrov talks, the outcome of the EU arms embargo talks, and for more details about the negotiations before we decide,” he said.

US Secretary of State John Kerry and Russian Foreign minister Sergei Lavrov discussed Syria in Paris yesterday, each backing different sides in the rapidly expanding civil war.

The Russians have secured an agreement in principle from the Syrian regime to take part in the planned Geneva 2 talks, while the SNC remains uncommitted.

An increasingly rancourous dispute over control of the opposition alliance has pushed the SNC to the brink of collapse in Istanbul over the past five days.

“If we do not solve our internal problems here and now there will be no coalition left to speak of,” an SNC member said.

What was supposed to be a brief conference dealing with a host of pressing issues has, instead, become an embarrassing display of internal politicking and inefficiency.

It has undermined the already threadbare credibility of the main opposition alliance, both inside Syria and in the eyes of its international backers.

An opposition organiser in Damascus said the SNC had missed a chance to make itself relevant, just as the Syrian crisis is dramatically escalating and a strong, united opposition front was needed.

“The fighting groups and the activists working inside Syria were demanding to be represented in the coalition but remain excluded,” he said.

“Since it was formed the SNC has done nothing tangible on the ground, it is a failure,” he said.

The timing could hardly be more critical.

Rebels are fighting against impossible odds in Qusayr, under a heavy assault from regime troops and Shiite militants from Hezbollah.

The European Union is currently considering lifting an arms embargo – which in theory would aid the supply of weapons  to the Free Syrian Army,  the SNC’s military wing –  and the opposition risks being outmanoeuvred on the diplomatic front after the Syrian regime indicated it would take part in the Geneva 2 peace talks, tentatively scheduled for next month.

Problems within the SNC came to a head in the early hours of yesterday morning

A secular grouping, led by veteran dissident Michael Kilo, together with other, smaller blocs, had sought to add 22 seats to the 63-member alliance.

The Kilo grouping is a liberal, broadly secular bloc that its supporters say would counter the dominance of the Islamist Muslim Brotherhood and help reassure the SNC’s international allies that it will safeguard sectarian and ethnic minorities.

But, in a 3am vote yesterday, after four days of deadlocked, at times angry, talks, just eight new seats were approved.

Six members from Mr Kilo’s bloc, were given places in the SNC – far short of the 25 it had opened the negotiation process with on Thursday.

Jamal Suleman, a popular actor, Anwar Badr, an Alawite writer and former political prisoner and Mr Kilo were all voted in.

Under SNC rules, existing members were required to vote on who should join and each candidate needed 42 ballots for approval.

Adding more than 20 seats would have significantly altered the balance of power within the SNC.

But entrenched members elected not to dilute their influence by opening the doors to what would have been another major bloc.

Broad Inclusion of Mr Kilo’s group would have weakened a widely held perception of excessive dominance   dominance of the Muslim Brotherhood, one major faction, and a grouping headed by the Qatari-backed SNC secretary general, Mustafa Sabbagh.

The SNC has a reputation even in opposition circles, as being controlled by the Muslim Brotherhood, with backing of Qatar and, to a lesser extent, Turkey.

In the days running up to the Istanbul conference – originally scheduled for three days but extended to six – a deal was reportedly brokered between Qatar, Saudi Arabia and the Muslim Brotherhood to significantly expand the SNC, in exchange for a serious injection of funding from international backers.

Western supporters of the opposition were also advocating for an expansion to counter the dominance of Islamists and improve the image of the SNC within Syria’s sectarian and ethnic groups.

That deal appeared to have broken down yesterday, amid furious recriminations.

“There was a shouting match between the French and one element of the SNC,” said an opposition member who was present.

“The French said ‘unless you expand you will get no support from any of us’, and was told ‘leave us alone, we don’t need your support’.”

International pressure on the SNC has, members said, been intense.

An SNC official described it as counterproductive.

“The Kilo group came with all these international supporters and Syrians don’t like to be put under that kind of pressure, it doesn’t work,” he said,

There were also signs last night that Mr Kilo’s bloc was beginning to fragment, with members saying they were representing themselves and talk of defections, according to SNC members involved in the talks.

By yesterday evening what was being called a “reconciliation” effort was underway, with members locked in a conference hall and told they could not leave until they had settled their differences.

Divisions have plagued Syria’s opposition since the start of the uprising in March 2011.

Moaz al Khatib, the former president of the SNC, resigned earlier this year, in part because of refusals within the alliance to widen its membership.