Five crew members have died on a cruise ship operated by the British holiday firm Thomson after a lifeboat fell from the vessel during an emergency drill at La Palma in the Canary islands, according to Spanish officials.

Three other crew were injured in the incident on the Thomson Majesty, a 20-year-old ship owned by Cyprus-based Louis Cruises and leased to Thomson, which was moored at the island's capital, Santa Cruz. Testing lifeboats has a history of accidents, and a seafarers' union said it advised members not to sit in lifeboats being lowered during drills.

Government officials quoted emergency services as saying crews were sent to the dock just after midday local time (12pm GMT) after "a lifeboat with occupants had fallen overboard from a cruise ship docked at the pier".

According to Spanish reports those killed were believed to be three Indonesians, a Filipino and a Ghanaian. Two Greek nationals on the lifeboat were seriously injured, with another Filipino slightly hurt. It is believed the boat dropped 17 metres into the water before overturning. It was not known what caused the boat to fall.

According to the report about 2,000 passengers were on board the Majesty, though none were involved in the drill. The ship is based in the port and usually operates cruises around the Canaries and Madeira.

A spokesman for Nautilus International, a trade union for seafarers, said lifeboat drills had a notorious danger record.

Andrew Linington said: "We've had this happen so often and the industry has moved lamentably slowly to deal with the problem.

"There's been research which suggests that more people are dying in lifeboat drills than are being saved by lifeboats. It's that serious. The death toll has been such that we advise our members: if you're doing a drill the drill is about raising and lowering the lifeboats. It shouldn't be about people actually getting into them. We advise them to do it without people in the lifeboats."

An investigation into the issue in 2001 by Britain's Marine Accident Investigation Branch found 15% of all deaths involving professional seafarers involved lifeboat drills, with 12 deaths over 10 years and 87 people injured.

Linington said the danger was caused by a variety of factors including the heights involved, corroded equipment being poorly maintained, unclear instructions and poor crew training.

Deaths at sea remained all too common, he said: "It's an inherently dangerous life, even in the 21st century, and we'd say too dangerous. For British seafarers, and they tend to be working at the higher end of the industry, the workplace death and injury rate is over 50 times what it is on average for all land-based workers."

In general, he added, safety on cruise ships tended to be better than in some sectors, but could still be patchy: "We would say there are still too many preventable accidents on cruise ships, and lifeboats is a classic example of that."

Most non-officers came from developing nations, particularly the Philippines or elsewhere in south-east Asia, Linington said: "It's certainly hard work. For the officers the pay is pretty good and the working conditions are hard, but for the ratings they work very hard.

"They're often away for eight or nine months at a time and do very long hours, and in many cases the work is very physical. It's a tough life. The pay can be good by the standards of their home countries but it's pretty low."

A spokesman for Thomson, owned by the German travel giant TUI, said: "Thomson Cruises can confirm there was an incident involving the ship's crew during a safety drill on board Thomson Majesty, in La Palma, Canary Islands today at 11:50am local time.

"We can also confirm that there have sadly been five crew fatalities and three crew members injured. One person has been discharged from hospital and we expect the other two people to be released from hospital imminently. Our thoughts are with the families of those involved.

"We are working closely with the ship owners and managers, Louis Cruises, to determine exactly what has happened and provide assistance to those affected by the incident.

"We are also working closely with all relevant authorities and will be co-operating fully with their investigations."