The European Parliament has “more power” over a British exit deal than any EU member state and will be ready to wield its veto, the assembly’s chief negotiator has said in a warning over the complexity of agreeing Brexit terms.

In an interview, Guy Verhofstadt, the former Belgian premier and leader of the parliament’s liberal bloc, staked a claim for his institution to be involved in talks from “from day one” and urged Europe not to force Britain into another “love-hate relationship” it regrets.

Although EU member states will play the decisive role in shaping a Brexit deal, the European Parliament, an unruly body of growing clout, must approve exit terms and any future trade deal. For Britain, that makes the need to cut deals with Mr Verhofstadt and other parliamentary leaders almost unavoidable.

“If the treaty says that we have to approve [an exit deal], the parliament has more power here than the individual member states,” Mr Verhofstadt said, referring to the fact they do not have a veto. “Right or wrong, this parliament has already refused things [and rejected draft international pacts].

“I do not have to be defensive, to say oh yes, we are important; everybody can read it in the treaty, so it’s not so difficult. So for once the treaty is very clear.”

Mr Verhofstadt’s devotion to a federal Europe and fiery oratory — his schoolmaster resorted to taping his mouth shut to stop him jabbering — have made him a bête noire of Brexiters. In Westminster, his recent appointment was met with a mix of alarm and mockery. “Get thee behind me, Satan,” joked David Davis, Britain’s Brexit minister, when asked about Mr Verhofstadt’s views.

There is no love lost: Mr Verhofstadt has called Brexiters “rats fleeing a sinking ship”, depicted the Leave campaign as “full of lies”, and said it would be “mad” to give Britain the single market advantages if it clamps down on EU immigration.

Yet in an extended interview, Mr Verhofstadt kept his sharper views in check, denying any ill will towards Britain, quoting Thatcher and Churchill and talking with pride about his vintage Aston Martin and Elva sports cars. “I like Britain,” he said. “I race British cars. How more a lover of Britain can you be than racing a British car?”

“You have to put everything within a clear, transparent, comprehensive, framework and I think in that sense Brexit is also an opportunity because maybe that could be the starting point or so to simplify our different relationships we have outside the union, as it is necessary inside the union,” he said.

He has long argued countries such as Britain that are unwilling to make the federal leap should have a “special associate status”. It is an arrangement — first outlined by the EU’s founding fathers in the 1950s — that may suit the British.

“Divorces are painful, at least painful . . . I was a lawyer in the past so I did a few of them,” Mr Verhofstadt said. “Instead of making this a painful divorce that weakens Europe and Britain, let’s try this time to put in place a system where everybody feels . . . happy is maybe too much, but comfortable. Is that not the right way?

“I want to avoid the mistake from the past where we give the impression that we push Britain in one or other direction,” he added. “That has to be avoided, I think, absolutely. Not to have this love-hate relation as we got for nearly 40 years.”

Behind the conciliatory tone, however, Mr Verhofstadt hinted at how difficult it might be for Britain to tailor an exit deal to its political needs.

Asked whether he could imagine the City of London remaining the euro’s financial centre after Brexit, he replied: “If [Britain] is fully fledged in the internal market and they accept the four freedoms, then OK. But then I ask myself: what was the reason to go out of the EU?”

His reference is in particular to the principle of free movement, which Britain is looking to restrict while preserving its trading rights. “Politically everybody had already said 100 times, 1,000 times, in every member state here in the parliament, in the [European] Commission, that we cannot undo the four freedoms.”

Mr Verhofstadt’s own career has charted the slow-motion separation of Britain and Continental Europe. Once known as “baby Thatcher” for his aggressive liberal views, Mr Verhofstadt hosted a joint Belgium-UK cabinet meeting with Tony Blair in Ghent.

That relationship however descended into acrimony over the Iraq war, with Mr Verhofstadt hosting the so-called “praline summit” of countries opposed. Against British objections he pushed for greater EU defence co-operation; a few years later Mr Blair duly vetoed his bid to become commission president.

Does he hold a grudge? Is he offended by being likened by British ministers to Satan? “From time to time you can use a joke to say what you think. British humour is . . .” he said, trailing off. “The proof will be in — how you say that? — in eating the pudding.”