London (AFP) - British Prime Minister David Cameron released his tax details Sunday in a bid to defuse a row over his father's offshore business, but faced further questions over money he received from his mother.

Cameron took the unprecedented step of publishing a summary of six years of his tax returns, and revealed that he got a Â£200,000 ($280,000, 240,000 euros) gift from his mother, on top of Â£300,000 in inheritance from his late father.

The 2011 gift from Mary Cameron raised questions about whether it was an attempt to dodge inheritance tax later down the line.

The release came a day after Cameron admitted he had mishandled the situation, telling party activists: "blame me", while hundreds of demonstrators rallied outside Downing Street demanding his resignation.

The gift was tax-free, and would only become liable to inheritance tax of up to 40 percent if Cameron's mother dies within seven years of handing over the money.

Cameron's Downing Street office said the gift came on top of the inheritance from his father, with his mother attempting to "balance" the sums received by her four children.

Cameron admitted on Thursday that he had held shares in his late father's Bahamas-based investment fund, which cropped up in the leaked documents from Panama law firm Mossack Fonseca.

He and his wife bought the stake in Blairmore Holdings for Â£12,497 in 1997 and sold it for Â£31,500, four months before he became prime minister in 2010.

The gift from his mother raised further questions about whether that money came in part from offshore funds.

"The week started with a moral crisis at the heart of the Tory party and has now ended with a scandal at the very top of government," said a spokesman for the opposition Labour party.

"The prime minister has been forced to admit that not only had he benefited from a company that paid no tax in 30 years, but that he may pay no tax on any benefits potentially gained from the same company.

"David Cameron can't hide any more. He needs to come to parliament on Monday and put the record straight."

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- Spotlight on British territories -

The revelations this week appeared to have undermined Cameron's claims to be spearheading a global clampdown on offshore havens.

But he insisted Sunday that Britain was at the "forefront of international action to tackle aggressive tax avoidance and evasion" with the announcement of a Â£10 million taskforce to scrutinise the Panama Papers.

The force will be led by Britain's tax authority and the National Crime Agency, and will investigate the leaked files from Mossack Fonseca to identify clients suspected of money laundering and tax evasion.

The revelations in the Panama Papers, resulting from what the law firm blamed on a computer hack launched from abroad, revealed how the world's wealthy stashed assets in offshore companies.

Cameron has sought to lead international efforts to improve financial transparency and plans to host an international anti-corruption summit next month.

Senior officials from German Chancellor Angela Merkel's governing coalition called on Cameron to clamp down on tax evasion in British offshore territories more decisively.

"We are continuing to press for more transparency but we can only be convincing on the world stage if we are first fully compliant in the EU," Ralph Brinkhaus, the parliamentary group deputy leader for Merkel's Christian Democrats, told Welt am Sonntag newspaper.

"For me, that also means that that Britain must exercise influence over its overseas territories. We have to make that clear to the Brits in upcoming talks."

Carsten Schneider, Brinkhaus's counterpart with the Social Democrats, junior partner in the right-left grand coalition, said Cameron needed to shore up his damaged credibility on the issue.

"If David Cameron still wants to be taken personally and politically seriously in the fight against tax fraud and tax evasion, Britain has got to close loopholes at home as soon as possible."