A security alert was sparked by a car fire last night as Prime Minister Tony Blair visited trade union leaders.

First reports said a car had exploded just metres from the Brighton Centre where Mr Blair was addressing the TUC General Council.

The Thistle Hotel in Kings Road, Brighton, was evacuated after smoke poured from the burning car into the building.

Scores of guests gathered in the street where emergency services had cordoned off seafront Kings Road and diverted traffic.

It is believed a fire started in a car parked in an underground car park next to the hotel and the vehicle exploded.

Fire, ambulance and police crews were sent to the Thistle Hotel shortly after 9pm.

Insp Ian Byford, of Sussex Police, said: "Nobody was injured and nobody was in the car.

"The registration has melted. It's unrecognisable.

"The force of the explosion caused car to move out of its parking bay.

"Fire service investigators are examining it to establish the cause of the fire."

Firefighter Mark Hodiak, of Preston Circus fire station, said: "There was thick black smoke coming from the car park entrance in Kings Road. It took 40 minutes to get the fire under control."

The incident took place close to the £3.7 million security island created around the Brighton Centre for the forthcoming Labour Party conference.

However, the blast did not disrupt the Prime Minister's speech as he pressed home the Government's tough message that trade unions have to modernise or face decline.

Mr Blair told the union leaders their membership had halved since 1979 and he pressed them to undergo "fundamental modernisation".

He said: "The blunt truth is that unions operate in the market in the same way as everyone else does." The Prime Minister acknowledged that union leaders were raising real issues causing workers hardship and he said he understood the "strong feelings" surrounding the Gate Gourmet dispute.

But he made it clear the Government was not going to allow a return to secondary industrial action, bluntly telling the union leaders: "It will not happen."