The shadow foreign secretary has urged Tony Blair to do more to help Zimbabweans forced to give up their British citizenship.

In a letter to the prime minister, Michael Ancram expressed concern at reports that the British High Commission in Harare is advising people that they will "categorically and irreversibly deprive themselves of any existing and future British rights" by giving up their citizenship.

In the past, British officials have taken into account that many renunciations of citizenship were not fully voluntary, said Ancram.

Ancram warned that if the reports were true "it would mean in practice that British citizenship is being forfeited under duress and that the UK government seems, on the face of it, to be helping Mugabe get his way".

He told the prime minister "it is essential that such allegations are addressed with urgency and hopefully fully refuted".

"Can you firmly assure those Zimbabweans who have had to renounce their British citizenship that you will see fit to allow them to reclaim their British citizenship status when necessary?"

The letter also expressed concern that the High Commission was charging for extensions to British passports at black market rates.

Ancram said he had been told that those paying in Zimbabwean dollars were being charged at and unofficial rate of around 1000 dollars to the pound, rather than the official rate of 90 to the pound.

The Conservative call came as a delegation of Zimbabwean politicians and human rights campaigners spoke at a news conference in the House of Commons.

One of those attending, John Huruva of the London-based Movement for Democratic Change, said the government had "failed to understand the misery faced by Britons forced to return from Zimbabwe and it is not doing enough to help their own citizens rebuild their lives".

"It beggars belief why the British government have been so reticent to help British citizens returning from Zimbabwe. Perhaps they fear that by offering proper assistance they will send a message that all UK citizens in Zimbabwe should come back to the UK.

"What the Government should do however is put real pressure on Mr Mugabe and his racist regime and help British citizens in Zimbabwe do what they want most - to stay on their farms and rebuild the country," he said ahead of the event.