George Galloway has been expelled from the Labour party after being found guilty of four of the five charges of bringing the party into disrepute.
On the fifth charge - urging voters in Plymouth not to vote Labour - Mr Galloway was acquitted.
The Glasgow Kelvin MP said he was "prepared" to stand as an independent MP, but was now "considering his position and consulting the [anti-war] movement".
But he refused to give an instant response to his expulsion from the party after 36 years membership, saying merely "I will definitely defend my position in parliament. I am not leaving politics".
He added that he "may have made his mind up" by the time of a public anti-war meeting to be held in London next week.
Outside the hearing, Mr Galloway told reporters: "This was a travesty of justice, a politically motivated kangaroo court. The verdict was written in advance.
"It is clear now that Mr Blair intends to go after Glenda Jackson and Bob Marshall-Andrews QC. His response to the disaster of the war is to attack those who opposed the war.
"Mr Blair launched these proceedings in an interview in the Sun with his muse, Trevor Kavanagh [the Sun's political editor]."
The MP said he would not join any other existing party, saying the Scottish National party and Scottish Socialist party were both in favour of independence, and he was not.
The Glasgow Kelvin MP must now decide whether to resign as an MP and force a byelection where he would stand as an independent, or stay on until the next general election without the Labour whip.
An added complication is that his seat is likely to be redrawn under boundary commission rules to reduce the total number of Scots MPs at Westminster before the next general election.
Mr Galloway told Guardian Unlimited he had been found guilty on the four charges so far on unanimous verdicts, despite his strenuous rejection of the allegation that he called on Arab armies to fight British troops.
He criticised the fact that the Labour party had decided not to release the transcript of today's hearing for another seven days.
Mr Galloway, who was prominent in both the parliamentary opposition to the war and many anti-war demonstrations, has been careful not to say he would stand as an independent if expelled by the Labour party, for fear of adding to the charges of disloyalty levelled against him.
But he did tell Guardian Unlimited in an online interview back in April: "If I am sacked, what message will they [the British people] take about the nature of Mr Blair's regime from that? If necessary I will defend the new Glasgow Central constituency on a platform of real Labour values, and I believe I shall win."
The charges faced by Mr Galloway were that:
· he incited Arabs to fight British troops
· he incited British troops to defy orders
· he incited Plymouth voters to reject Labour MPs,
· he threatened to stand against Labour
· he backed an anti-war candidate in Preston.
But in a 50-page deposition to the NCC tribunal, Mr Galloway argued he was being singled out for punishment for voicing legitimate opinions shared by many in the party, including former cabinet members Robin Cook and Clare Short.
At the opening of the tribunal yesterday, Mr Galloway dubbed it a "political show trial", saying it was more appropriate to the regime of Saddam Hussein in Baghdad than to modern day Britain.
The former Labour MP Tony Benn gave evidence on his behalf, pointing out that the prime minister had described peace protestors as having "blood on their hands".
"Serious things were said on both sides," Mr Benn told reporters.
There is no internal appeal mechanism against the expulsion.
Mr Galloway said he would now be addressing a series of public meetings at Labour party offices up and down the country, starting in Wood Green tonight "if they let me in".
He said he was deeply sad to leave the party he joined aged 13. He said the Labour party would "rue the day" it expelled him, quoting the examples of Ken Livingstone, Denis Canavan and Rhodri Morgan.
Rose Burley, chair of the national constitutional committee panel, said in a statement: "Following the case brought by the national executive committee of the Labour party to the national constitutional committee and after a two day hearing the unanimous decision of the panel of the national constitutional committee found four of the five charges brought against Mr Galloway proven and the decision of the panel was that Mr Galloway be expelled from membership of the Labour party forthwith."
The Stop The War coalition, which mounted protests outside the committee meeting in support of Mr Galloway, said the expulsion was an "absolute disgrace".
The convenor of the coalition, Lindsey German, said: "George Galloway told the truth before, during and after the war with Iraq, whereas Tony Blair has told nothing but lies.
"It is disgraceful that the Labour party is penalising George Galloway and giving Tony Blair a standing ovation because that does not reflect the British people's views."
Geoff Martin, London convenor of Unison, said: "This will drive more people away from the Labour party and will make it more difficult to maintain the link between the party and trade unions."
The socialist campaign group of Labour MPs immediately called for an emergency meeting of the NEC to overturn "this contemptible decision and reinstate George Galloway with immediate effect".
Mr Galloway told Abu Dhabi TV that the war in Iraq was illegal and urged British troops not to obey "illegal orders".
And he asked: "Why don't Arabs do something for the Iraqis? Where are the Arab armies? We wonder when the Arab leaders wake up? When are they going to stand by the Iraqi people?"
A further charge related to a speech Mr Galloway made in Preston, Lancashire, following the May 1 local elections, in which it is alleged he congratulated anti-war Socialist Alliance candidate Michael Lavalette on winning a formerly safe Labour seat on the city council.
Another stemmed from an address to 600 people in Plymouth in January where, the party claimed, he exhorted his listeners not to vote for Labour candidates unless they heeded anti-war sentiment.
And the fifth related to claims that he was planning to stand as an independent against Labour in the next general election if he was expelled from the party.
But none of the charges relate to the story - now subject to libel proceedings - in the Daily Telegraph that Mr Galloway received over £300,000 in payments from the Saddam regime.
The Labour party chairman, Ian McCartney, said: "The NCC is a body that is fierce in its independence.
"Its deliberations came in direct response to an unprecedented number of complaints from party members and members of the public.
"The issue here is a very simple one. George Galloway incited foreign forces to rise up against British troops at a time when they were risking their lives.
"He was the only Labour MP to do this and he has never taken back or apologised for these comments.
"Any reasonable person would have been disgusted by this incitement and I believe the NCC reached the right verdict today."