It is "very difficult" for the EU to function as a group of 25 states under its current rules, Tony Blair has said.
Mr Blair: Rule changes needed
But he urged people not to focus on the failed constitution which had been supposed to set up new structures.
The prime minister was being asked by the Commons liaison committee how many member states the EU could cope with under its present format.
Rule changes were needed to make the EU work effectively, but enlargement had "done Europe a power of good", he said.
He conceded that there was some scepticism over proposals to further enlarge the EU, but he believed reforming nations wanting an "open" Europe were winning the argument.
However, he said: "I think it's very difficult for Europe to operate at 25 with its present institutional structures, in particular the six-month revolving presidency.
"Having said that, I think enlargement has done Europe a power of good."
Pressed about further enlargement, he said: "There is a mood in Europe about enlargement that's not altogether positive but I think it's important we try with other allies to turn that around."
He said most people were "pretty realistic" about the stalled constitution.
'Non means non'
"We have had two 'nos' in Europe, from France and Holland, and unless those are reversed, that's the position," he said.
"We have already agreed we will revisit the position but let's not return to focusing on that exclusively."
He said the big issues facing the EU were the state of its economy, organised crime and illegal immigration.
"Let's focus on that and then come back to the constitutional questions," he said.
There was also a "real worry" over the border security of new countries joining the EU, he added.