Duke and Duchess of Cambridge
The royal couple will stay at the Taj Mahal Palace hotel in Mumbai which was targeted by terrorists in 2008 Chris Jackson/Getty

The Duke and Duchess of Cambridge have been urged to cancel their trip to India just days ahead of a royal visit to the country. It comes after police in Punjab issued an alert saying that terrorists may have crossed into India from neighbouring Pakistan.

Concerns were growing after authorities said Mumbai, Delhi and Goa could be targeted by extremists. Will and Kate are due to visit India's financial hub as well as its capital city. The Sun quoted an Indian security source as saying: "We are not taking this threat lightly. We cannot rule out attacks."

Police are on the hunt for a Swift Dzire car used by the terrorists. According to reports, the vehicle is packed with explosives and possibly even a suicide belt.

The royal couple will stay at the Taj Mahal Palace hotel in Mumbai, which was bombed by Islamic militant group Lashkar-e-Taiba during a series of coordinated raids in the bloody 2008 attacks which left at least 164 people dead, and the worrying development has led to calls to scrap the visit.

"They need to cancel the visit. It's not worth the risk," said John O'Connor, former head of the Met's Flying Squad. "If there was an attack, even close to the royals, it would been seen as a major coup for IS (Isis) and Co.

"The royals need to stay away from any hot spots," he added.

According to The Sun, one security source said: "The Indian security services are not a patch on the UK's but they'll be the ones responsible for protecting the royals." The unnamed individual added: "It's also unlikely that the Met royal protection officers will be armed, so they'll be totally reliant on local security."

Police have urged the Indian public to remain vigilant. "You are directed to take all the necessary steps in this regard and specially focus on strict checking of vehicles, security of all vital installation including police buildings and defence establishments, religious places, crowded places such as market areas, malls, railway stations, railway tracks, and educational institutions," the alert said.

The Duke and Duchess of Cambridge will kick their tour of India off in the bustling metropolis of Mumbai on 10 April where they will honour the victims of the 2008 attacks. They will then head towards the capital, New Delhi, followed by the state of Assam in northeastern India where they will visit the Kaziranga National Park, a world heritage site that contains two-thirds of the world's greater one-horned rhinos.

The royal couple – who are travelling without Prince George and Princess Charlotte – will then pay a visit to the Himalayan mountain kingdom of Bhutan before returning to India where they will end their trip with a tour of the majestic Taj Mahal.