Jean-Claude Juncker, president of the European Commission, insisted that only a long extension would be offered

Britain will be unable to block any increase in its payments to the EU in the event of a long extension to the Brexit negotiations.

At a summit next week EU leaders will offer Theresa May a choice between a no-deal Brexit and a long Article 50 extension unless the withdrawal treaty is ratified by April 12.

A long extension, until January 1 2020 or as late as the following year, will come with conditions including guarantees that Britain will not use its EU voting weight to block key decisions.

“A long extension will be conditional on loyal membership obligations,” a European diplomat said.

The prime minister will be asked to make a written commitment that the government would effectively abstain on important decisions on