London (CNN) The campaign to prevent Britain leaving the EU, such as one exists, is not being led by a charismatic politician from Britain, nor even by wistful Eurocrats in Brussels. Instead, the standard-bearer of the anti-Brexit charge -- at least while the UK Parliament is on its long summer break and Prime Minister Theresa May is walking in the Alps -- is a little-known former political apparatchik vacationing on a Mediterranean island at the other end of the European Union.

Until a couple of months ago James Chapman was chief of staff to David Davis, the UK's Secretary of State for Exiting the European Union and the man in charge of Brexit negotiations for Britain. Before that, he worked for pro-EU former finance minister George Osborne, and for a long period he was political editor for the stridently pro-Brexit Daily Mail newspaper.

But now Chapman, from his holiday villa on the Greek island of Spetses, has put rocket boosters under the Brexit debate with a warning delivered on social media that the UK's withdrawal from Europe will be a "catastrophe" and that May risks driving the British economy "off a cliff" in the process.

Chapman has not had a sudden, retsina-inspired conversion about the biggest issue facing the UK -- he is known to have buried his pro-European sympathies in his previous jobs. And his Twitter tirade might have remained just that: a firestorm celebrated in liberal echo chambers and and largely ignored beyond them.

But Chapman added an irresistible zinger: a suggestion that Foreign Secretary Boris Johnson and fellow Brexit supporters might have been jailed if British electoral law outlawed the publication of misleading statements. (Brexit campaigners have abandoned a notorious claim during the 2016 referendum campaign that leaving the EU would mean the UK could afford an extra £350m a week on its National Health Service.)

His strongly worded broadside against Brexit and its supporters galvanized those who are still holding out for Britain to remain in Europe.

And, in the dog days of August with little else to write about beyond the drama of North Korea, the British media took the bait.

Call for new anti-Brexit party

Chapman found himself all over the papers, and, reveling in the notoriety, went on to call for a new anti-Brexit political party, bringing pro-EU elements of British politics together under the name "The Democrats."

That catapulted him onto the BBC's flagship morning news program Friday, where, in a feisty interview, he claimed the brand of Theresa May's Conservative party has been so damaged by Brexit it will fail to win an outright majority ever again, and that both the Conservatives and the main opposition Labour party have been taken over by their extremist fringes.

Chapman's incendiary language has enraged pro-Brexit Conservative MPs, who think he's deliberately making mischief as his old boss prepares to enter the third round of delicate Brexit negotiations with the EU in Brussels later this month. (They may have a point -- Chapman has signed up to write a regular column for the pro-EU Guardian newspaper and all the publicity is doing his profile no harm.)

But his intervention has also won support from other Conservative MPs -- including, Chapman claimed on Friday, two Cabinet ministers.

With the Labour party led by the left-wing Jeremy Corbyn, and the Conservatives seen as increasingly right-wing under May , there have been serious discussions in Westminster about a new centrist party for at least a year -- even before the EU referendum result.

Brexit has given that centrist idea a new pro-European dimension, because both Corbyn and May are in favor of a so-called "hard" Brexit, with the UK severing all ties with the EU including the single market. This centrist movement has support in particular from Britain's smaller parties like the Liberal Democrats, who found themselves squeezed at the election called by May in June , when there was no overall winner between Conservatives and Labour. Chapman also claims there would be popular support for such a party because 48% of people voted to remain in the EU.

MPs reluctant to break away

But the problem with the idea of "The Democrats" is that no serious politician in either Labour or the Conservatives is yet prepared to jump ship -- to leave the comfort of a well-established party structure, with its members, activists and donors, to the unknown territory of an entirely new movement. Getting into bed with one-time political foes would be a huge leap, no matter how solid the common ground on the big issue of Brexit.

What is true is that MPs from different parties have been working more closely together on a range of issues, including Brexit. Last month, a new cross party commission was created -- the All-Party Parliamentary Group on EU Relations , whose members include senior MPs who are opposed to a hard Brexit, including the former Conservative minister Anna Soubry and the one-time Labour leadership contender Chuka Umunna. They are working together as negotiations continue in Brussels and, once the UK Parliament reconvenes in the autumn, will try to soften the Brexit legislation that will repeal EU laws and bring them back under British control.

Britain is on course to leave Europe in March 2019 -- and, even though Davis, the Brexit Secretary, has had a difficult time in negotiations with his opposite number Michel Barnier, this is still the most likely scenario. But the anti-Brexit movement sees the parliamentary process, using cross-party cooperation and tabling amendments to the repeal legislation, as the best opportunity to stop a hard Brexit.

Such an all-party group allows its members to oppose Brexit without the high risk move of leaving their own parties to join an entirely new one.

The problem with this strategy so far is that such a cross-party group, by its nature, has to work slowly and with consensus. What those who are concerned about Brexit -- including voters -- needed was someone to get angry on their behalf. The disparate anti-Brexit movement needed galvanizing into an outraged, forthright and indignant force. Chapman has not become its leader, but its outrider. It is not without irony that it has had to come all the way from a Greek island.