Zambia's former ruling party has been stripped of its legal status for not paying fees for the past 20 years - amounting to $75,000 (£50,000).
The Registrar of Societies says the Movement for Multi-Party Democracy (MMD) has been deregistered for failing to pay its "statutory obligations".
The party says it does not owe anything and will appeal against what it brands "an assault on democracy".
If the decision is upheld, the MMD will lose its parliamentary seats.
The MMD is one of Zambia's oldest political parties, sweeping to power in 1991 in the country's first multi-party elections.
Last year, it became the largest opposition party with 53 seats when it lost power to the Patriot Front of President Michael Sata.
All political parties in Zambia are obliged to register with the Registrar of Societies, a government body.
"I have cancelled the registration of the MMD for non-compliance with the Societies Act," Clement Andeleki, head of the agency, told a news conference in the capital, Lusaka.
"This drastic decision has been taken as an act of great exception by my office to send strong signal to erring registered societies," he said.
He said there had been attempts to get unpaid fees from the MMD since September when President Sata took over.
The president used to be a leading member of the MMD before forming his own party.
MMD spokesperson Dora Siliya said the party had paid its dues - as recently as January this year - and no correspondence had been received on the matter.
"We believe this is all political and an assault on democracy in this country," Ms Siliya told the BBC's Focus on Africa programme.
"We take this with great exception that without evidence, or indeed any correspondence, that such a drastic decision can be taken with a very large opposition party," she said.
"We believe we have been paying our statutory obligations - and if some of our branches have not been paying, which I hear may be the case, they could be deregistered, not the mother body, which under Zambian law, can be registered and continue to exist."
MMD lawyers would be launching a judicial review of the decision, Ms Siliya added.
The MMD has 21 days to appeal to the Minister of Home Affairs.
If the MMD loses its appeal, the speaker of the National Assembly has the right to declare the party's parliamentary seats vacant - and by-elections will be held.
News of the deregistration came as former President Rupiah Banda said he was stepping down as leader of the MMD, which he announced he would do after losing last year's election.
The MMD said the move was not related to the deregistration.