Mr McMillan-Scott said he had been made a scapegoat
Conservative MEP Edward McMillan-Scott has threatened to sue the party for expelling him in an escalating row over the Tories' new European alliance.
He was thrown out after defying the party and standing for vice-president of the European Parliament against a fellow member of the Tories' EU group.
He has accused the Tories of sitting with extremist figures in Brussels.
He said he was still "committed" to the party but warned David Cameron may head the most Eurosceptic government ever.
Mr Cameron has come under fire over Europe in recent months after dropping a pledge to hold a referendum on the Lisbon Treaty after it was ratified by all EU members.
Mr McMillan-Scott was suspended and subsequently expelled from the party after successfully challenging Polish MEP Michael Kaminski for an influential role within the European Parliament.
Mr McMillan-Scott had been unhappy with Mr Cameron's decision to re-align the party in Europe, which he had said he would do ever since becoming Conservative leader in 2005.
Mr Cameron pulled his party out of the European People's Party, where it sat with governing parties in Germany, Italy and France, saying the group's federalist views were incompatible with his party's stance on Europe.
Instead, it formed a new coalition, the European Conservative and Reformist Group, made up of centre-right parties from central and eastern Europe.
Mr McMillan-Scott said Mr Kaminski, a member of the new coalition, was "inappropriate and unsuitable" for the role because of reported past links with extreme groups, something the Polish MEP denies.
Now sitting as an independent Tory, Mr McMillan-Scott maintains his expulsion was unfair and is now considering legal action against his party in both the UK and Europe.
"There is no shame in losing the whip on a point of principle," he said. "To be expelled for the same thing was disproportionate and against natural justice."
The MEP, who once headed the Tory grouping in Brussels, said the Conservatives were "no longer the party I knew".
He called for prominent eurosceptic MEPs who had defied the party leadership over the Lisbon referendum issue to be similarly disciplined.
"David Cameron risks leading the most eurosceptic government ever to take office in one of the EU's largest and most powerful countries.
"Pro-European voices have been stilled of late and now I intend to speak out."
The Conservative leadership said it had written to the MEP outlining the steps he would need to take to have the whip restored, including resigning the post of vice-president of the European Parliament, and he had failed to respond.
Opposition parties say the Tories' new alliance puts them on the fringe of Europe but the Conservatives say they will continue to work with other groups in the Parliament in the UK's national interest.