Britain and America will today remember those who died in US terrorist attacks a year ago.

Tony Blair will join other senior members of the government, the Royal family, foreign diplomats and families of the UK victims for a service of remembrance at St Paul's cathedral.

Number 10 stressed that the arrangements for the service - co-ordinated by Tessa Jowell and officials at the Department of Culture, Media and Sport - have been agreed with the families of the 67 UK victims.

The culture secretary and the US ambassador, William Farrish, will both read lessons at the gathering.

In New York, the Speaker of the Commons, Michael Martin, will lead the UK delegation at the main service of commemoration in Battery Park.

He will also be meeting firefighters and members of other organisations that were involved in the response to the destruction of Manhattan's Twin Towers.

Last night, the US administration placed America on "orange" alert, following "specific and credible threats" that al Qaeda planned new attacks.

Defence secretary Geoff Hoon will join US defence secretary Donald Rumsfeld at the Pentagon for a separate service commemorating last year's Washington victims.

A series of memorial ceremonies will be held in London, many highlighting the UK's close links with America.

The American Church in London is expected to hold a service of remembrance and a "One year on" service will also take place at St Bride's Church in Fleet Street to remember the journalists and photographers who died.

In the City, the London Metal Exchange, The London international Financial Futures and Options Exchange and Lloyd's of London will hold a minute's silence.

In Brussels, the European Union will mark the anniversary with flags, anthems and choirs in the entrance to the European parliament.

A ceremony will then take place inside the parliamentary chamber attended by leaders of Europe's political groups, MEPs, EU officials and the US Ambassador to Brussels, Rockwell Schabel.