The Take That frontman worked with the musicals mogul on the official Diamond Jubilee single, 'Sing', last month - which was released to celebrate Britain's Queen Elizabeth's Diamond Jubilee - and its success has inspired them to collaborate again.
Gary has already sent Andrew an outline for one new song and they're planning to continue writing together on holiday in Spain this summer.
Andrew said: ''I am writing with Gary Barlow again. I've come up with the tune this time and he's already emailed me with lyrics this morning. It hasn't got a title yet.
''Gary's going to be down in Majorca this summer and so will I, so we'll get together and something will come out of it.''
Andrew says he's pleased with the song they've got an idea for, but he has no idea who will record it.
He said: ''I'm quite pleased with this little song we've done, I'm not sure who we'll give it to - maybe nobody will want it?''