Brown and Merkel in talks

A Downing Street spokeswoman told the BBC that the talks would probably focus on "international issues" and refused to say whether Europe would be discussed.

But she said "they will discuss the key issues facing them both."

The talks between the two come as Mr Brown is facing increased calls for a referendum on the next EU treaty by the opposition Conservatives.

The Conservatives argue that the treaty, set to be finalised by the end of the year, is similar to the rejected EU constitution.

The Labour government had promised to hold a referendum on the constitution.

Adding more pressure to Mr Brown this week, a recent ICM poll for the Daily Mail suggested that 82% of voters in the UK want a poll on the treaty.

For her part, Ms Merkel is one of the key reasons there is a proposed new Reform Treaty to replace the constitution, given the thumbs down by French and Dutch voters in 2005.

Germany was in charge of the EU for the first half of this year and the chancellor used the time to cajole other leaders into accepting an outline for a new treaty.

The treaty contains several of the innovations of the constitution but has introduced opt-outs in key areas for the UK.

It has also dropped references to symbolic statehood issues such as the anthem and the flag.

The resulting treaty is an awkward compromise that keeps enough of the original so as not to upset those member states that had already ratified the treaty but allowed France's Nicolas Sarkozy to say that his country will go through parliamentary ratification rather than opting for a public poll.

Ireland is the only country that has definitely said it will have a referendum. However, if Britain follows suits it will probably mean that others will also be politically pushed to take the referendum route.

This would likely upset the careful timetable, first outlined by Ms Merkel, that would see the new treaty in place by mid-2009.