LONDON (Reuters) - A senior European Union official has cautioned Britain against seeking an "a la carte" membership of the European Union, warning that London should not dismantle what has already been signed up to.
Michel Barnier, the European commissioner in charge of overseeing the EU's single market, told Reuters that while it was possible for countries to opt out of certain EU projects, it could not pick and choose from existing agreements.
"A multi-speed Europe is something different from an a la carte Europe," Barnier said in an interview. "It doesn't mean that some take bits and pieces of what's been established so far and move off in different directions."
The former French foreign minister made his remarks after delivering a similar message in London's Mansion House at the heart of Britain's financial district, where he warned that the country risked losing its status as a "gateway" to Europe were it to leave the EU.
His comments come after British Prime Minister David Cameron recently pledged to claw back powers from Brussels then hold a referendum on whether the country should stay in the EU at all.
Instead of changing existing commitments, Barnier said it would be better for London to stay outside future EU schemes it disagreed with.
"In future, we won't always move together at the same pace on the same subjects," he said.
"There can be opt-outs ... on Shengen, on currency. Either we move ahead together ... or on certain subjects ... some move ahead as pioneers," he said, citing examples such as the decision by 11 of the 27 countries in the European Union to pursue a tax on financial transactions.
Britain, which is not in the bloc's common currency, has already opted out of a fiscal pact to tighten budgetary control within the euro zone and, more recently, a system of bank supervision led by the ECB.
Barnier's message, however, may fall on deaf ears in some parts of eurosceptic Britain.
His critics have accused the Frenchman, who is also in charge of writing new EU financial laws, of driving a continental agenda to overhaul finance, shackling the City of London.
Barnier is currently examining possible reforms to the structure of banks in Europe.
He said he was focusing on trading activities and that a far-reaching revamp was possible. That could come in a proposal for legislation by the middle of this year.
"We are examining all the options. I am looking at a particular part of banking activity which can raise risks - market making," he said.
"We are going to engage in a proper reform," he said, adding that he wanted changes that would also "keep the diversity of the banking sector".
This week, Germany unveiled plans to reform banks which fall short of physical separation of activities. France has also signalled reluctance to break up its big universal banks.
Barnier also said there was merit in considering whether plans to impose losses on bondholders in failed banks from 2018 could be brought forward. This is a measure designed to get to grips with sick banks, without calling in taxpayer money.
Barnier visits Washington in two weeks for talks with regulators and the Federal Reserve on ironing out differences between new EU and U.S. rules on banking, derivatives and markets.
"There are a few points where there is a risk of divergence and we are not ready to accept divergence," Barnier told journalists earlier.
The EU is watching how new U.S. derivatives rules will affect the cost and ability of EU firms to do business there.
The Fed has proposed requiring foreign banks to group their subsidiaries under a holding company, subject to the same capital standards as a domestic bank.
"If the Americans decide to do certain things, we will do the same sort of thing," Barnier said, adding that although he would be "constructive" in his talks with Washington, he was not "naive".
Editing by David Holmes and David Cowell