Prime Minister Gordon Brown could miss the signing of the EU Reform Treaty next week because of a diary clash, Downing Street has said.
He is due to face the Commons liaison committee - made up of senior MPs - on Thursday morning, just hours before the ceremony takes place in Lisbon.
A Number 10 spokesman said: "He does want to go... it is hoped he can go."
Foreign Secretary David Miliband is likely fly to Portugal to sign the document if Mr Brown is unavailable.
The EU Reform treaty has proved highly politically controversial, with the Tories arguing that it is substantially the same as the abandoned EU Constititution, on which a referendum was promised by Labour.
But the government says the two documents are different, making a public vote on the treaty unnecessary.
Precedents
Mr Brown has been invited to Lisbon by the Portuguese government - which holds the EU presidency - along with the leaders of the other 26 EU member states.
Downing Street has denied that Mr Brown wants to avoid the ceremony.
A spokesman said there were precedents for prime ministers not signing key EU Treaties, pointing out that the then foreign secretary Robin Cook signed the Nice Treaty in 2001.
But most of the other national leaders are expected to be in Lisbon next week.
The EU Constitution was thrown out in 2005 after it was rejected by Dutch and French voters.
The treaty, once it is signed, still has to be ratified by the UK and all other member countries before coming into force.
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