The crew has already encountered heavy storms during the trip

The four, led by skipper Mark Stubbs of Poole, Dorset, have only just recovered from rough seas that set off their emergency Mayday beacon.

They have also encountered a freak wave during their 55-day bid to row 2,100 miles (3,380km) from Canada to Britain.

The crew's weather adviser says a large gale could strike the team on Sunday.

By Sunday, this could result in the worst wind and sea conditions encountered so far Lee Bruce

Recent bad weather set off the team's Emergency Position Indicating Beacon while their carbon-fibre boat, Pink Lady, was at anchor.

A week ago, they were forced to go 45 miles (73km) out of their way to avoid heavy storms, while in the first days of the challenge they encountered twice as many icebergs as usual off the Canadian coastline.

Their next hurdle is to overcome what could be their worst weather conditions since setting off on 30 June, the team's weather expert, Lee Bruce, has said.

They have just 420 miles of the challenge to complete.

Mr Bruce said Hurricane Alex, currently causing havoc in the Atlantic, will die down in the next few days but its tail end could combine with another low pressure system.

He said "It [Hurricane Alex] may then combine with another low-pressure centre as it nears Pink Lady, creating a large gale with its centre near the boat.

Charity aim

"By Sunday, this could result in the worst wind and sea conditions encountered so far, lasting through Monday and threatening to force the men into their cabins to ride out the dangerous conditions."

Firefighter Mark Stubbs, 40, from Poole; ex-SAS diver Peter Bray, 48, from Bridgend; journalist Jonathan Gornall, 48, from London; and digital mapping specialist John Wills, 33, from Farnham, are battling to pass Bishop's Rock Lighthouse.

They need to pass the Scilly Isles' landmark by 23 August to break the world record for the fastest North Atlantic row from Canada.

They are then due to head up the Channel towards Falmouth to land at the town's National Maritime Museum.

They are hoping to raise �50,000 for the British Heart Foundation.