Seventy Nepalese builders working in Qatar in the runup to the 2022 football World Cup have died on construction sites since the start of 2012.

Fifteen have died this year, according to a death toll announced by Nepal government representatives in Doha. It is the clearest official data yet on the dangers facing 1.2 million migrant workers in the Gulf kingdom during the $100bn (£62bn)construction drive before the World Cup and came as David Cameron called on Qatar's leadership to take action. He said zero deaths on the London 2012 Olypmics project showed Doha "it can be done".

Nepalese trade unions said many of the fatalities were caused by workers without proper safety equipment toppling from the upper floors of buildings.

The death toll was released at a joint press conference held by the governments of Nepal and Qatar, at which they denied Guardian reports about brutal working conditions, long hours, lack of food and pay and squalid living quarters facing Nepalese workers.

Mohammad Ramadan, a legal adviser working for Nepali nationals in Qatar, claimed all Nepali workers were "safe and fully respected" but he cited data from the Nepal embassy, which revealed that 20% of the 276 Nepalis who died in Qatar last year were killed on building sites. The rest died of natural causes and in accidents not at the workplace. This year 151 Nepalis have died, one in 10 on building sites. Sources said workers had also been killed walking on Doha's congested roads and from heat exhaustion and dehydration.

There are 340,000 Nepali workers in Qatar and if the mortality rate was extrapolated across all migrant workers it would suggest that more than 200 foreign workers could have died on Qatari building sites since the start of 2012.

"This reminds us of the industrial revolution 150 years ago," said Sharan Burrow, secretary general of the International Trade Union Confederation. "Young healthy men are being worked to death in Qatar. Scores are dying from heat exhaustion and dehydration after 12-hour shifts in blazing heat, often during the night in the squalid and cramped labour camps with no ventilation and appalling hygiene."

Last week the Guardian reported that documents showed 44 Nepalese workers died in Qatar between 4 June and 8 August this year, and that more than half died of heart attacks, heart failure or workplace accidents. It said evidence of exploitation and abuses pointed to "modern-day slavery, as defined by the International Labour Organisation".

Ali bin Samikh Al-Marri, chairman of Qatar's national human rights committee, said information cited by the Guardian was false and the numbers exaggerated.

"There is no slavery or forced labour in Qatar," he said. "There have been some problems, owing to the fact that there are 44,000 businesses in the country. But I can assure you that the authorities are constantly making efforts to resolve the problems."

The joint press conference came 48 hours before a Fifa executive discussion in Zurich on workers' safety on 2022 World Cup projects, and three days after Nepal withdrew its ambassador to Qatar. She had previously described Qatar as an "open jail" for Nepalis who suffer labour abuses.

Nepal has the second-largest migrant workforce in Qatar after India and its economy relies heavily on money sent home by its migrant workers. In 2012 the World Bank calculated that remittances accounted for 22% of Nepal's entire economic output and the figure is rising.

Bishnu Rimal, Gulf co-ordinator of the General Federation of Nepalese Trade Unions, said workers' deaths ruin families in Nepal who rely on remittances and were deeply distressing, with repatriation of bodies taking as long as three weeks.

"Qatar should enforce its own labour inspection system," he said.

Qatar's minister of labour, Saleh al-Khulaifi, said on Sunday that the Gulf state would recruit more inspectors to mount raids and checks on companies to ensure they complied with labour laws and hired more interpreters to speed up the handling of complaints from foreign workers, as part of what the ITUC said was "a major damage-limitation exercise".

In Manchester on Tuesday Cameron urged the Qataris to improve their safety record. "My message is that they ought to insist on better," he told BBC Radio 5 Live. "We, in the Olympics, I think I'm right in saying, managed to build that entire Olympic Park with the best ever record on safety – no one dying during construction, keeping injuries to an absolute minimum. It can be done."