Ghana's main opposition party has reacted with anger after one of its most influential leaders was sentenced to 20 years in jail, with hard labour, for illegal mining offences.

The New Patriotic Party (NPP) alleged that the landmark judgement against Bernard Antwi Boasiako, the party's chairman for the Ashanti region, was politically motivated.

Popularly known as "Chairman Wontumi", Boasiako is the first senior politician in Ghana to be convicted of illegal mining since the government began a crackdown on the practice in 2017.

As well as receiving a jail sentence, Boasiako, who had denied the charges, was ordered to pay more than $10,000 (£7,450) in fines.

He looked visibly shocked when the judge handed down the sentence in a court in the capital, Accra, on Monday.

The NPP politician was found guilty of "assigning mineral rights without approval" and "facilitating an unlicensed mining operation" through his company, Akonta Mining.

Ghana is Africa's leading gold producer, but a large portion of the country's waters and environment have been polluted by illegal mining, known as locally as "galamsey".

Over the years, governments have attempted to tackle the problem, even deploying the military, to little effect.

Following Boasiako's conviction, NPP general secretary Justin Kodua Frimpong told reporters: ''The fight against illegal mining cannot become credible only when it targets political opponents in respect of whom evidence of wrongdoing does not exist."

He alleged that the judge had ignored "clear evidence" that would have exonerated Boasiako.

Prosecutors say the NPP politician had allowed two individuals, Henry Okum and Michael Gyedu Ayisi, to undertake mining activities on the Akonta Mining's concession without approval from the minister of lands and natural resources.

Environmental campaigners welcomed Monday's conviction, saying it will serve as a deterrent to other politicians and businessmen financing the activities of illegal miners.

"It sends a strong signal that accountability is indispensable if Ghana is to protect its forests, rivers and other natural resources," Adib Saani, executive director of the Jatikay Centre for Human Security and Peace Building, said.

However, Frimpong said the NPP's lawyers had started the process of challenging Boasiako's conviction.