ABUJA (AFP) – Britain on Friday threw its weight behind Nigeria's acting president after the surprise return from a hospital abroad by ailing President Umaru Yar'Adua sparked fears of a fresh leadership crisis.

Yar'Adua discreetly arrived in Nigeria on Wednesday, throwing the country into confusion and reviving concerns of a leadership squabble just two weeks after his vice-president Goodluck Jonathan was installed acting president after a leadership vacuum of nearly three months.

The former colonial ruler joined a chorus of concerns that Yar'Adua's return would not plunge Africa's most populous country, the world's eighth biggest oil exporter, into uncertainty regarding who was in charge.

"It is important to avoid uncertainty at this time," British Ambassador Bob Dewar said in a statement.

The United States and local political activists have voiced fears that the return of the frail Yar'Adua would could be a ploy by his loyalists to sideline Jonathan and stir instability in the west African giant.

There was a flurry of statements from government on Thursday to reaffirm Jonathan as being in control, after Yar'Adua returned from Saudi Arabia, where he had been receiving treatment for a serious heart condition.

Jonathan was referred to as vice president in the first statement from Yar'Adua's office, which confirmed the ailing president's return, but said that his deputy would remain in charge while Yar'Adua continues to recuperate.

Dewar said Britain welcomed the official clarification on leadership.

Opposition activists have also expressed concern that Jonathan appeared to have been sidelined in the preparations to fly Yar'Adua back.

Dewar said the British government continued to encourage Nigeria to act consistently with "the constitution and the principles of democracy, good governance and the peaceful rule of law".

"It is also important at this time to avoid any political or constitutional confusion that could put at risk the integrity and transparency needed in the conduct of public affairs," he said.

Yar'Adua had been in Saudi Arabia since November 23.

It was not clear Friday whether the two leaders had met following Yar'Adua's return to Nigeria.

Jonathan himself moved to reassure the 150 million Nigerians on Wednesday saying "the ship of state is on course" while Yar'Adua continues to recover.

Britain urged Jonathan to quickly push ahead with the many "pressing priority issues" affecting Nigerians such as combating corruption and poverty, enhancing governance and democratic accountability, and bringing peace and stability to the oil-producing Niger Delta.

"The British government will support the acting president and government in taking these and other issues forward in a spirit of constructive partnership," said Dewar.