The high-profile ceremony will take place between 12:00 and 13:00 - just two hours after the UK leader starts a session before senior parliamentarians in the committee.

According to a Downing Street spokesperson, Mr Brown "certainly hopes to go to Lisbon", as "he negotiated the treaty, he is the head of the government and takes full responsibility".

The spokesperson also dismissed the claims that Mr Brown was trying to avoid the ceremony. "There are lots of precedents for heads of government not signing", he said, according UK daily The Guardian.

In 2001, then foreign secretary Robin Cook signed the Nice treaty.

The Portuguese EU presidency, hosting the event, is expecting "all" EU leaders. "From what we know, he [Mr Brown] is going", the spokesman told The Guardian on Wednesday (5 December).

The bloc's Lisbon Treaty, a set of new institutional rules agreed in October, has been a highly controversial project in the UK, with Mr Brown under constant pressure to put it to a referendum.

The opposition Conservatives are the main driving force behind the call, claiming the newly-agreed treaty barely differs from the draft European Constitution rejected by French and Dutch voters in 2005.

Mr Brown argues that the UK has secured special arrangements - including an exemption from the Charter of Fundamental Rights and the justice and home affairs area - which prevent London from giving more powers to Brussels.

After being formally signed by all European leaders in Lisbon, the treaty will go for ratification next year. It is supposed to come into place by mid-2009, ahead of the next European elections.