One of Britain's biggest cranes rolled into Bradford last night to put a new roof on to John Street Market.

But traders said they were amazed to receive a letter from a Council officer asking them "not to venture into Bradford" in the evenings while the work took place.

The letter from Bradford Council says: "I would like to take this opportunity to ask traders not to venture into Bradford during the evenings next week to view the crane in operation, however tempting this may be."

One trader who did not want to be named said: "It's way over the top telling us not to venture into Bradford at night. I couldn't believe it when I read the wording."

But Richard Meade, of John Street Market Traders Association, said: "It is a massive operation and I can understand that they wouldn't want sightseers for health and safety reasons.

"The crane work is very welcome, because it is one of the biggest parts of the redevelopment."

Malcolm Veigas, head of the markets service, said: "The letter to traders is aimed at insuring every necessary health and safety arrangement is in place for this major and complex lifting operation."

The crane is capable of lifting 50 tonnes and will haul ten 35-metre roof sections into position above the existing roof.

Later, the old roof will be removed and replaced with a curved structure to complement the state-of-the-art market hall.

The use of the crane will involve the closure of Rawson Road and Northgate until 6pm next Tuesday. Traffic will be diverted via John Street and Westgate. The crane will suspend work during the day to lessen disruption for shoppers and traders.

The £3.5m refurbishment is expected to be finished before Christmas and traders have continued to operate during the work.

The John Street traders will be joined by stall holders from the temporary Rawson Market and James Street Fish Market.

John Street market will receive a massive refurbishment, transforming it into a modern shopping centre.

Council leader Councillor Margaret Eaton said: "This is an exciting sight of the major work now under way to create impressive new shopping facilities at John Street Market. It is a key element in the future redevelopment of the northern side of the city centre.