BBC's Alistair Jackson: The government wants to ban the trade

The Dover service - due to start on Friday - has been called unethical by campaigners.

The ports and airports from which British calves and sheep were sent abroad to be slaughtered were the scene of violent protests three years ago.

One campaigner, Jill Phipps, was killed outside Coventry Airport when she fell under the wheels of a livestock lorry she was trying to stop.

Jill Phipps: Died in a protest at Coventry

Attempts to ban the export of animals failed after the High Court ruled that port authorities could not refuse to handle the trade.

The ban on beef exports means that calves are no longer exported, but 12,000 sheep a week still leave Britain.

The trade is worth up to £200m a year to farmers and they insist all their procedures comply with strict guidelines to make sure the animals suffer as little as possible during the journey.

Campaigners Compassion in World Farming (CIWF) said 440,000 lambs and sheep were exported to the Continent last year and the new ferry service will push the 1998 figure even higher.

Flashback to protest in Plymouth

The CIWF said many of the animals going out through Dover are likely to be sent on long journeys to southern Europe to be killed in abattoirs that use cruel slaughter methods.

Often they will be carried in overcrowded trucks, which can lead to animals getting bruised and injured and, if they fall, trampled on by their companions.

The government is keen to stop the exports, but says banning them would breach European law.

Peter Stevenson, CIWF's political and legal director, said: "We urge Britain's sheep farmers even at this 11th hour to think again and abandon the cruel live export trade.

The BBC's Alistair Jackson reports

"In trying to escalate this trade, farmers are flying in the face of public opinion which is increasingly concerned about animal welfare, flouting the government which has stressed its strong opposition to live exports, and ignoring scientific research which has concluded that journey times should be kept to a minimum."

But a spokesman for the National Farmers' Union said: "The NFU is concerned to ensure that livestock producers are able to utilise all trade to secure markets for their livestock.

Dover - scene of the protest

"The loss of any market, particularly at a time of such crisis in the rural economy, threatens the very future of Britain's sheep farmers.

"Farmers care about the welfare of their livestock. All livestock exports are subject to the most stringent regulations which are enforced by MAFF."

Juliet Gellatley, founder of vegetarian charity Viva!, said protests could reach the same level as previous protests when thousands of people disrupted ferry sailings at Dover.

She said: "More people than three years ago are willing to come out and do this.

"Their resolve is very strong. People are prepared to stand in front of lorries. That is the bottom line."