City forecasters have warned today that fears of a potential Brexit have started to take their toll on the UK’s economy. The most recent data obtained by City analysts shows activity in the services sector plunged last month to its lowest level in three years. If the effect persists, it would be a “hammer blow” to the growth of the British economy.

Following the survey, economists have negatively readjusted the British economy’s growth prospects for the coming months. According to analysts, businesses have started to get jittery about a potential British exit from the European Union.

“Survey responses reveal that firms are worried about signs of faltering demand, but boardrooms have also become unsettled by concerns regarding the increased risk of Brexit, financial market volatility and weak economic growth at home and abroad,” Chris Williamson, chief economist at Markit, explained.

The figures already account for the month Boris Johnson announced his intention to side with the Out campaign. They come on the heels of the Mayor’s angry reaction to a French minister’s assertion that, in the eventuality of a Brexit, bankers in the UK would quit London for Paris.

The Mayor hit back that “There’s a reason that London is ‘the fourth biggest French city on Earth’ and it isn’t our membership of the EU. It is because London is by far the financial, cultural and innovation capital of Europe and Project Fear won’t change that fact,” he concluded.

For weekly news updates, subscribe to our email newsletter

Related: NOA says Brexit is no good for outsourcing, but Britain needs reformed EU membership