German Finance Minister Wolfgang Schaeuble said at a business conference in London that the European Union would be less stable and less competitive if the United Kingdom decides to leave the organisation.
"If the UK would not be engaged (in) the European Union as a member, I think the UK would take the risk that continental Europe would be a little bit less stable, more volatile and that is not in Britain's interests," Schaeuble said, speaking in English.
Schaeuble was speaking alongside his British counterpart George Osborne at the British Chambers of Commerce (BCC) annual conference in London.
He also said that Britain made a big contribution to the EU with its expertise in security and foreign policy.
Osborne said it would be a mistake for Britain to leave the EU at a time of geopolitical instability and risks posed by developments in Russia, the Middle East and North Africa.
"If we were to leave the EU, over 50 of our trade deals with other countries in the world would automatically fall because they are trade deals with the EU," he added.
Osborne also warned against the argument that Britain would retain a special status on trade if it leaves EU.
He said there was a risk it could have "the worst of both worlds" by having to pay into the EU budget without having a say in setting the rules.
Schaeuble said Britain was a force for improving the competitiveness of the bloc, adding that he did not want to lose an ally in the fight to reduce unnecessary rules and regulations.
Asked what he would do if Britain voted to leave the EU, Schaeuble joked "We would cry... and I hope that we will not."
British Prime Minister David Cameron and Osborne back Britain's continued EU membership, but their ruling Conservative Party is deeply divided on the issue and several cabinet ministers support a "Brexit."
The UK will have an in-or-out referendum on EU membership on June 23. Opinion polls say the result could be very close.