At the heart of the dispute is Mr Blair's intention to do away with Labour's historic constitutional cornerstone, Clause IV, which commits the party to large-scale nationalisation. Substantial left-wing opposition, it emerged, is entrenched among Labour's MEPs: more than half were named in a newspaper advertisement last week as opposing reform, on the very day that Mr Blair made his first official visit to Brussels.
Mr Blair's anger at being upstaged was matched by an extra-ordinary attack on him and his allies from Ken Coates, MEP for Nottingham, in language rarely heard in political debate. He said: "I just cannot tell you my contempt for those shits."
The former miner and founder of the Institute for Workers' Control added: "Bugger the next election. What difference is it going to make if we have Clarke or if we have Blair? The unspeakable Mr Blair has no scenario other than if he could only get into bed with the Liberals he could be in government for ever and ever."
Glenys Kinnock, MEP for South Wales East and wife of the former Labour leader, retorted yesterday: "It was the views that Ken Coates continues to promote that took our party close to an abyss.''
The dispute, reminiscent in its ferocity of the early 1980s, is indicative of the tensions, faction-fighting and reservoir of traditional socialist sentiment that are still commonplace with Labour in Brussels and Strasbourg - where the European Parliament sits.
Labour MEPs are not just fighting the Westminster leadership, it became clear yesterday: they are fighting among themselves. Some of those whose names appeared on the newspaper advertisement calling for Clause IV to be retained feel their views were misrepresented. They are accusing the hard left Campaign Group of taking advantage of a round-robin signed by 32 MEPs last October to embarrass Mr Blair without them realising it was happening.
Thirty-two of them were named in the advertisement, but several later claimed they had no warning about it, although its organiser, Alex Falconer, MEP for Mid Scotland and Fife, argues that all were notified twice ahead of the meeting which agreed to itsplacement.
The following day, seven of the same MEPs appeared to switch sides, joining a total of 36 colleagues who signed a pro-Blair letter. Hugh McMahon, MEP for Strathclyde West and one of the seven who apparently switched, said his fears had been mollified since original reservations in October and he accused the Campaign Group of ambushing Mr Blair. He said: "Factionalism has been revived by the Campaign Group, the traditional Tribunites and some of the Euro-federalists who adopted this to attack Mr Blair.''
That, however, is only part of the picture which emerges from the Byzantine politics of Strasbourg and Brussels. One source described the apparent retraction as "the habitual cock-up factor''. MEPs, he added, "are trying to hunt with the hounds and run with the hares. They want to buy off the trade unions and convince their Greek socialist colleagues they are hard men of the left - while keeping their relationship with the leader. It is known as the Brussels posture: facing both ways."
Because Labour's centre of gravity at Strasbourg is well to the left of the party at Westminster, several MEPs were keen to be associated with the anti-Clause IV campaign. They were not so happy, however, to embarrass Mr Blair as Labour's best electoral hope so openly.
The manoeuvring is difficult for outsiders to comprehend because the composition of the Euro Labour group is also very different from that at Westminster. It includes a large, but not homogeneous, group of left-wingers, a smaller number of modernisers, Euro-federalists and anti-Europeans.
Geoff Hoon, a former MEP and moderniser, now MP for Ashfield, said: "When I joined the group in 1984 there was an awful lot of political infighting, with slates being formed, round-robins signed, for example, committing people to being anti-European. I had a very bad time to begin with. Being pro-European and wanting to win the next election, I was not popular."
The left, though powerful, is not cohesive. The Campaign Group influence extends to several prominent MEPs, such as the organiser of the advertisement, Mr Falconer, who later attacked Mr Blair as a "Leninist'', Alf Lomas (London North East) and Eddy Newman (Greater Manchester Central).
The Tribune Group is much larger but less focused. According to Mr McMahon "the majority of members would be Tribune Group but it is not cohesive. It consists of fellow-travellers of the Campaign Group, anti-marketeers, and people like me who are critical but constructive about Europe and are determined to make the best of it." There are, for example, left-wing anti-marketeers, ranged against colleagues who believe Euro-federalism is the only route to Socialism. Friendships cut across ideological divisions: a group of "pro-marketeers'' dines every Monday night when in Strasbourg.
Many left-wingers have been in place since the early 1980s, the high tide of the Bennites. Take, for example, Stan Newens, (London Central) : the former Westminster MP and biographer of Nicolae Ceausescu, the fallen dictator of Romania, who was elected in 1984. Such members, according to one moderniser, "missed the Kinnockite changes - let alone the Blairite ones".
Mr Falconer's perspective is different. He argues that MEPs "are much more radical than MPs at Westminster because there, half of the party are front-bench spokespeople and are therefore stifled in debate".
The other side of this lack of patronage, however, is that the discipline imposed by the Chief Whip is missing, as is the scrutiny of the media - which leaves the Strasbourg parliament largely unreported. With few worries about the reporting of what theysay, political judgement is often suspended. As one critic put it: "When they get on that plane to Brussels they seem to breathe different air. To put it bluntly, they're in space.'' The size of their constituencies means a campaign to unseat an MEP is difficult to organise. Meanwhile their numbers have swollen.
Because of the scale of Labour's victory at the polls last year, some of those elected were thought to be fighting hopeless seats and would not have stood a chance of winning a Westminster seat. Those who made it have to accept being second in the political pecking order to their Westminster colleagues. Mr Hoon recalls the MEPs' battle to gain access to Westminster, which culminated in him being apprehended by a burly policeman on his first visit. The Commons authorities had issued him with a builder's pass.
Mr Hoon believes that MEPs should be brought into the debate through greater interaction with the Commons. More typically, one Labour MP said: "In the great sweep of history, in the story of the reform of Clause IV, the MEPs will be lucky if they register as a micro-dot.''
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