Britain’s exit from the European Union could cost the UK almost 500,000 jobs and nearly 50 billion pounds ($68 billion) in investment by 2030, according to a study commissioned by the mayor of London.

In a worst-case scenario, Britain would lose 482,000 jobs and 46.7 billion pounds ($62.9 billion) in investments in the next 12 years, according to research published Thursday by Cambridge Econometrics.

The report compares four possible post-Brexit scenarios to the option of remaining in the bloc, which has already been ruled out by the British government.

The most optimistic Brexit scenario outlined, of a two-year transition period leading to single market membership without the customs union, would still lead to a loss of 176,000 jobs and 20.2 billion in investment.

London Mayor Sadiq Khan, who campaigned against Brexit and commissioned the study, said the findings show "the potential economic risks and human costs at stake in the negotiations" in Brussels.

In a statement, the pro-EU mayor accused the government of a "complete lack of preparation" in assessing the consequences of Brexit.

Khan said it was “astonishing” that British Prime Minister Theresa May’s government had failed to do any analyses.

“This new analysis shows why the government should now change its approach and negotiate a deal that enables us to remain in both the single market and the customs union,” he said.

Transition arrangements, trade and security will be on the agenda of Brexit negotiations later this year before Britain leaves the bloc in March 2019.

The EU has slashed UK’s economic growth forecast for 2018 and said the slowdown would continue through 2019 as uncertainties over Brexit weigh on the economy.

Brussels believes that the British economy will slow to 1.3 percent in 2018 and 1.1 percent in 2019.

The figures put Britain's economy lowest in the bloc with Italy, and far behind the growth rate predicted for the euro area, which is expected to be 2.2 percent this year and 2.1 percent in 2018.