German chancellor may have an unexpected ally in David Cameron, the U.K. prime minister.

Even as opposition grows to Angela Merkel's campaign for an overhaul of the European treaty, the German chancellor may have an unexpected ally in David Cameron, the U.K. prime minister.

When Mr. Cameron meets Ms. Merkel for talks in Berlin on Friday, he is expected to offer to back limited changes to the European treaty in exchange for guarantees that the U.K. will continue to have an explicit opt out from tighter fiscal rules. Mr. Cameron also wants guarantees that none of the changes Ms. Merkel envisages will hurt London as a global financial center, according to U.K. officials familiar with the matter.

Mr. Cameron and his finance minister, George Osborne, have openly backed the need for greater euro-zone integration, even if it requires treaty change. Berlin's more ambitious changes—if they help stabilize the euro zone—could therefore be quietly welcomed in Whitehall, where the knock-on effects of the euro zone's troubles on the U.K. economy are the major concern.

But there are two catches, U.K. officials say. First, Britain would want explicit recognition that it would not be bound by tighter fiscal rules—extending the opt-out it currently enjoys in Brussels for running large deficits. If it gets this, it could help the government argue there is no need for a referendum on the treaty changes, officials say. The U.K. coalition government has written into law a pledge to hold a vote if further U.K. powers are transferred to Brussels. Second, it also wants safeguards to make sure that as euro-zone integration tightens it wouldn't be locked out of key decisions on the single market or on regulatory decisions that would affect the City of London.

Ensuring London's special status outside the euro-zone could be the way for Berlin and Brussels to gain Mr. Cameron as a powerful ally in their bid to revamp the European Union. A show of support from Mr. Cameron would be a welcome sign for Ms. Merkel, who has faced a week of constant criticism of her drive for treaty change from visiting leaders ahead of the Dec. 9 European Council meeting.

Ms. Merkel faced fresh opposition to the plan on Thursday amid warnings that euro-zone integration without involving the broader EU could be driving a wedge between the 17 euro-zone members and the remaining 10 EU members who aren't in the currency club.

Members outside the euro zone are growing increasingly concerned that the 17-member currency bloc is distancing itself from the other 10 member states in the broader EU. Many of the decisions taken to combat the euro-zone debt crisis, which also have a direct impact on the whole EU, are taken by a small group of euro-zone officials.

Concerned about the growing split, Danish Prime Minister Helle Thorning-Schmidt said after meeting Ms. Merkel in Berlin that the EU as a whole must deal with the crisis and, in another blow for Ms. Merkel, rejected the chancellor's pleas for wide-ranging changes to the European treaty. "In the crisis we are now facing, it is perhaps more important that the 27 member states stick together," Ms. Thorning-Schmidt said at a joint news conference with Mrs. Merkel. "It is exactly when we have a crisis that we should stick together as the European Union, use our institutions and use the community method."

Irish Prime Minister Enda Kenny was in Berlin for talks on Wednesday and also rejected Ms. Merkel's bid for treaty changes, saying the existing European treaty provides all the tools Europe needs to deal with its debt crisis.

Mr. Cameron has expressed concern that a growing split between euro-zone members and the other 10 EU countries will leave the U.K. frozen out of decision making on key issues like financial regulation. He is concerned that if the euro-zone countries obtain greater fiscal-policy union, the bloc could form a caucus within the EU to make changes at the expense of the member states outside the common currency. To assuage these concerns, Mr. Cameron may support Ms. Merkel's bid for treaty change if the U.K can receive protection of its interests.

Speaking at a conference before her meeting with Ms. Thorning-Schmidt in Berlin on Thursday, Mrs. Merkel acknowledged that there were "certain tensions" between the euro states and those outside the currency club. After the meeting, Mrs. Merkel sought to calm any fears of a split in the EU.

"Even though the euro member states surely have to resolve a few problems among their 17 member states, we will never forget that the 27 belong together, that the internal market is the foundation upon which the euro can work," she said during the news conference in the chancellery.

Mrs. Merkel wants changes to the European treaty that would allow far-reaching integration of fiscal policy within the euro zone to match the currency union. Germany also wants tough, automatic sanctions against euro-zone members that violate the rules on debts and deficits contained in the Maastricht Treaty on Economic and Monetary Union.

"We need to assure that we don't endanger the healthy part of Europe through the part that currently has its hands full to survive," Ruediger Grube, chief executive of Deutsche Bahn, the German railway, said. "The idea of sharpening the rules certainly isn't wrong."