"I think England are going to be a tough team to beat," he said. "They seem to have got their swagger back against us in that final Test match (England won by 247 runs). What we saw in that Test match is a team that when they are firing they are an incredibly tough team, not just to beat but to compete against."
He added: "I thought for long periods of the summer we held our own against them but when it really mattered they flexed their muscles. I don't expect that will change in the home summer here against Australia."
In comparison to Australia, who are still coming to terms with the shock axing of their coach Mickey Arthur on Monday, England can so far boast a relatively trouble-free preparation for the Ashes opener at Trent Bridge, which starts on July 10.
Kevin Pietersen, back in the England fold after three months out with the knee injury suffered in New Zealand, was denied his only chance for a hit against international opposition following Thursday's damp squib. But the 33-year-old appears in good form following his unbeaten 177 for Surrey at the weekend.
England's only possible point of contention appears to be who will join skipper Alastair Cook at the top of the order. Joe Root looks set to displace Compton, who played a starring role during the tour of New Zealand, hitting back-to-back centuries, but his place has come under scrutiny after he managed just 39 runs in four innings in the return series.
While McCullum acknowledged that Root - who broke into England's ranks as a Yorkshire opener - has the skill to make the move up the order, he believes England will have acted too hastily should Compton lose his place.
"It's probably a touch harsh on Compton after scoring two brilliant hundreds against us back home. For us we came off a Test series where he performed exceptionally well against us," he said. "We targeted him as one of the key wickets at the top of the order. He pained us for a fair amount of time back home with his innings.
"I thought we bowled exceptionally well to him (in England). They weren't easy batting conditions either at the top of the order. We'll wait and see how it unfolds from the openers' point of view in the Ashes."