David Miliband last night became embroiled in the row over Prince Charles's trip to America next week to pick up an award for championing green issues.

The Environment Secretary, asked about the 7000-mile round airline trip to Philadelphia and New York on which the prince will be supported by a 20-strong entourage, joked: "Was it a particularly heavy award? A lot of business can be done by telephone and videolink these days."

The implied criticism follows accusations of "green hypocrisy" against the prince made by the environmental lobby. However, Mr Miliband was keen to praise Charles's green credentials: "He has not been afraid to go to America and say to President Bush that climate change is an issue that needs to be addressed and I admire him for that."

No 10 sought to distance itself from Mr Miliband's remarks with Tony Blair's spokesman saying he was content to leave the matter to the Secretary of State's office.

During the two-day US visit with the Duchess of Cornwall on January 27 and 28, Charles will be carrying out other engagements, but the focal point will be the ceremony at which the prince will receive the Harvard Club's Global Environmental Citizen Award from actress Meryl Streep and Al Gore, the ex-vice president, who is now best known for his own green campaigning, including the film on climate change, An Inconvenient Truth.

Travel costs for the trip, which is in addition to Charles's main spring tour, will be met by taxpayers. It comes 14 months after the prince and the duchess last visited the US and four months before the Queen undertakes a state visit there in May.

Charles and Camilla are planning to use scheduled flights to get to the US instead of the usual method of charter flights. It was reported the prince has booked the entire first and business class sections of a jumbo jet but aides insisted they would book only 20 seats if they did travel scheduled.

A Clarence House spokeswoman said: "Receiving the award is part of a two-day visit, which will also highlight urban redevelopment and youth regeneration. They are going to the US at the request of the Foreign Office."

In December, Charles, 58, proud of his label as "the green prince", promised to reduce his carbon footprint by introducing changes to his lifestyle. Instead of using private jets and helicopters, he pledged to use the less carbon-consuming scheduled flights and trains.

He was due to take delivery of Jaguar cars adapted to run on biodiesel fuel. His homes at Birkhall on the Balmoral Estate, at Highgrove in Gloucestershire and at Clarence House in London would be run using green electricity. It was also reported that the prince had told his staff to make more use of bicycles.

In the past, Charles has been stung by criticism of his spending on royal travel. In 2005, a trip to the US cost the taxpayer more than £500,000. The travel bill alone for the week-long trip came to £330,000 after the royal couple flew to New York on a private jet. An earlier trip to Sri Lanka, Australia and Fiji by chartered plane cost £300,000.

Ian Davidson, Labour MP for Glasgow South West who sits on the Commons Public Accounts Committee, said: "This certainly does seem to require reporting to the committee in due course. Any suggestion that the Prince of Wales is travelling with his wife to America at enormous cost with a huge entourage to collect a green award would seem to be a contradiction in terms."

The back bencher said, as he understood it, it was once again a case of the prince travelling on an international flight to collect a green award and padding out his programme with other events to justify the expense and extravagance of another foreign journey. "This does not sit well with the green image that the prince is trying to project," he added.