Manfred Weber, the head of the centre-right European People's Party, the largest political grouping in the European Parliament, said the loss of blocking powers would be "compensation" for concessions that Cameron has demanded.
Cameron met several European leaders including Weber last week to push for reforms ahead of a national referendum on Britain's EU membership, including an opt-out of the closer union clause plus stricter requirements for migrants in claiming welfare payments.
"If there is a request on the table to change this phrase or to make an opt-out for Britain... then the rest of the European Union can ask for a compensation on this," Weber told reporters in Brussels.
"If Britain wants to have opt out on this question then the rest of the European Union can also ask, ok that means for the future that you are losing your blocking situation, your veto system on this question," Weber told reporters.
He added: "The logic behind (this is) if I insist on getting out of this ever closer union approach than I cannot have the right in the future to block others if they want an ever closer union.
"If Britain insists on this idea to have a protocol, then the rest of Europe should write in this agreement that they are not using their veto in the future - in any domain."
Weber later said on Twitter that this would only apply to areas in which Britain already has an opt-out, such as the Schengen passport-free zone.
The EPP is the party of European Commission chief Jean-Claude Juncker, who met Cameron in Britain last week to discuss the reform plans following the British premier's re-election last month.
Weber added that during his meeting with Cameron he urged him to bring "realistic" demands to a summit of EU leaders in Brussels on June 25-26.