By Laurence Peter
BBC News, Brussels
The parliament's powers would also change under the Lisbon Treaty
The European elections next month may lead to the creation of 18 "virtual" Euro MPs, who will not take office until the Lisbon Treaty takes effect.
The treaty has already been ratified by most of the 27 member states, but the Republic of Ireland is expected to hold a second referendum on it in October.
Lisbon can only come into force if all have ratified it - and Irish voters rejected it last June.
The 736 seats contested in this election will become 754 under Lisbon.
But even the figure of 754 is a transitional one, because the treaty sets the final number at 751.
It was agreed that Germany would keep its total of 99 MEPs until the next European elections, in 2014, when its number would fall to 96, in line with the Lisbon Treaty.
Political tussle looms
Spain, which is in line to get four of the extra 18 seats under Lisbon, is "pushing for a solution to advance the arrival of these people", a senior European Parliament official told the BBC.
LISBON TREATY PROGRESS
Approved by parliament: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, The Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, UK
Defeated by referendum: Irish Republic
Challenges: Legal objections delaying ratification in Germany, Polish president also delaying ratification, Czech president yet to sign treaty
In this election Spain has just one national list - a national constituency - so the job of selecting another four MEPs should be relatively easy, according to the official, who declined to be named.
But the situation is "far less clear" in countries where voters can choose individual candidates on party lists, or where there are several regional constituencies, as in the UK, the official explained.
After the June election there could be rivalry between regions and parties to get extra MEPs sent to Brussels. But it all depends on what happens to the Lisbon Treaty.
This election is being held under the terms of the Nice Treaty, which set the number of MEPs at 736 - down from the current 785.
The parliament official said it was possible that the extra 18 MEPs would get observer status in the parliament, once the Lisbon Treaty takes effect.
But the official said no formal decision had been taken about their salaries or expenses.
"In recent years all the observer MEPs were members of national parliaments, so they got the national parliament's salary. The European Parliament just paid the real cost of their travel expenses," the official said.
During the EU's enlargement, MEPs from new member states had observer status between the signing of the accession treaty and their country's actual accession date.
It will be up to the new parliament to decide the transitional arrangements for the extra 18 MEPs, the official said. They could get observer status if Lisbon takes effect in late 2010 or in 2011.
But a decision of the European Council - the member states' governments - would be required for them to take up their full duties before the next European elections in 2014.
This year a new salaries and expenses system is coming into force, putting all MEPs on a monthly EU salary of 7,000 euros (£6,160; $9,790). On top of that, their monthly allowance - for office costs and travel - will be 4,052 euros.
Under the Lisbon Treaty, France, Sweden and Austria will get two extra MEPs each, while the UK, Italy, Poland, the Netherlands, Bulgaria, Latvia, Slovenia, and Malta will each gain one MEP.
EU foreign ministers could agree at one of their regular meetings to give the 18 MEPs their full status before the 2014 elections, the parliament official said.
"If it's an amendment to a treaty, or a transitional measure, it requires an intergovernmental conference (IGC). But the difference sometimes is only in the wording. In the last half-hour of their meeting the foreign ministers can say 'this is an IGC'," the official explained.
The Lisbon Treaty's progress is further complicated by the fact that legal "guarantees" for Ireland, covering sovereignty, neutrality and some social issues such as abortion, are being bolted onto the treaty. This extra text then also has to be ratified by all member states.