Prince and Blair at Madrid service

Prince Charles and Tony Blair are to attend a memorial service in Madrid on Wednesday for the victims of the 11 March train bombs, Downing Street said today.

The news came as Scotland Yard urged the British public to be extra-vigilant over possible terrorist threats - and to use a special phone hotline to report the most trivial suspicions.

More than 200 people died in the four blasts on commuter trains in the Spanish capital, for which Islamist terror group al Qaeda has claimed responsibility. The UK security effort has been embarrassedby the success of two Greenpeaceprotesters in scaling Big Ben unchallenged on Saturday.

After attending the Madrid service with the prince at 11.30am, Mr Blair will hold meetings in Spain before travelling on to Lisbon to visit the president and prime minister of Portugal.

It is not yet clear whether, in Madrid, the Prime Minister will meet outgoing premier José Maria Aznar or his successor José Luis Rodriguez Zapatero, an outspoken critic of the Iraq war who has denounced Mr Blair and President Bush for "lies" about the reason for the conflict.

In London, anti-terror police called on landlords and hotel managers to watch out for suspicious tenants or guests, for motor traders to report suspicious customers and for bank staff to report attempts to set up possibly bogus accounts.

Shoppers and commuters were asked to report anyone paying unusual attention to security measures at shopping centres and stations to the hotline on 0800 789321.