Longtime senior Conservative Party grandee — and former chancellor of the exchequer — Kenneth Clarke (pictured above) in no uncertain terms yesterday said that a British exit from the European Union would be a disaster.

That Clarke is pro-Europe is certainly not a surprise.

As former prime minister John Major’s chancellor from 1993 until the fall of the Tory government in the 1997 Labour electoral landslide, Clarke was the most prominent pro-European in Major’s government — at one point, Clarke was even in favor of the United Kingdom joining the eurozone.  When Major’s government irreparably fractured over divisons on the UK’s role with respect to Europe, Clarke was most certainly the top general of the pro-European faction.

So it’s not a shock to see Clarke joining forces with Peter Mandelson, the former Labour veteran, and others for a cross-party effort to boost the United Kingdom’s continued presence in the European Union:

“There’s a broad range of opinion inside the [Conservative] party. The number of people who actually want to leave the European Union; it’s quite tiny. They get a disproportionate amount of attention. My guess is that there are about 30 who want to leave and when we first joined the European Community I think there was slightly more than that.”

He warned that it would be “pretty catastrophic” if Britain left the EU and said he was now resigned to fighting a referendum on the issue if the Conservatives win the next election.

“The background climate in this county has become … unremittingly hostile. I think somebody has got to make the positive case again. The climate of public opinion just needs to be reminded how essential it is if we really want the UK to play a part in the modern world,” he said.

But it’s another headache for UK prime minister David Cameron, who announced in a widely anticipated speech last week that he would seek to renegotiate the United Kingdom’s role in the EU and, thereupon, call a referendum on the UK’s continued membership by 2017 (obviously depending on the reelection of the Tories in the 2015 general election).

Clarke’s outspoken support shows just how difficult Cameron’s balancing act on Europe has become — and it will only be more difficult as a potential referendum approaches.

Clarke, whose Westminster experience dates to 1970, came of age as a minister in the era of Margaret Thatcher, reaching the pinnacle of power as Britain’s top financial minister under Major, where he received much of the credit for getting the British economy back on track after the 1992 ‘Black Wednesday’ crisis that forced the British government to pull the pound out of the European Exchange Rate Mechanism (a forerunner to monetary union and today’s single European currency).

While the Tories were in opposition throughout Labour’s subsequent decade-plus reign, Clarke remained one of its most popular officials, though the party thrice refused him the leadership — in no small part due to his enthusiastic views on Europe.

Clarke, as justice minister from May 2010 until September 2012 (he’s now just a ‘minister without portfolio’), has most recently pushed modernizing reforms to the UK approach to imprisonment, arguing that huge prison populations and ever-stricter sentencing were not necessarily the best approach to crime.

In many ways, he’s the heir to Edward Heath and his uniquely pro-Europe brand of ‘One Nation’ Toryism.  With Thatcher’s elevation in the 1980s, however, the Conservative Party pulled toward a more fundamentally conservative and eurosceptic orientation.

Cameron and Clarke themselves have an odd relationship.

Although Cameron defeated Clarke for the Tory leadership in 2005, they both come from a more modernizing approach for the Conservative Party.  In many ways, Cameron has shifted his party to the center — on everything from climate change to gay marriage to, at least initially, supporting Clarke’s efforts on a new approach to justice.

In calling for a renegotiation and a EU referendum, Cameron obvious hopes to please (or at least quiet) his eurosceptic base in the Conservative Party and hold off the even more anti-Europe United Kingdom Independence Party.  Moreover, one of his first moves upon becoming party leader in 2005 was to pull the Tories out of the larger Christian democratic and pro-integrationist European People’s Party at the European Parliament level, in order to sit with the more eurosceptic European Democrats.

But while Cameron’s not nearly as pro-Europe as Clarke, it’s equally obvious that Cameron doesn’t exactly want to be the prime minister who pulls the UK out of Europe.  Even Clarke believes that it’s inconceivable that Cameron would lead a campaign for a ‘Brexit.’  Indeed, if Cameron wins reelection in 2015, and if German chancellor Angela Merkel wins reelection later this year, it’s widely assumed that Cameron will renegotiate additional opt-outs on justice, immigration or other EU-level laws, in exchange for acquiescing to Merkel’s hope for a new treaty that creates greater political and fiscal union among the more exclusive group of eurozone nations (which, obviously, excludes the UK).

But by calling a referendum — to be held, perhaps, in five years, contingent on his winning the next election — Cameron risks a chain of adverse events that could well result in ‘Brexit’ and a half-decade of uncertainty for the United Kingdom.

Business interests — above all, financial interests in the City of London — would be aghast if the United Kingdom were suddenly excluded from the single market, and the United States has firmly indicated that it opposes any UK move to exit the EU.

So while Cameron has most immediately felt political pressure from the eurosceptic right, there will certainly be a fair share of pressure from the more pro-Europe center as well — and Tories like Clarke will certainly stick around through the next five years to remind Cameron of that.