Leading Zeros lost in csv format

I have an excel spreadsheet with numbers and the format has to show zeroes at the beginning (e.g. 0011111) It is always going to be a seven digit number. I can format the cells to show the numbers with the leading zeros, but as soon as I save the file as a csv, the leading zeros are lost.

Does anyone know how to the zeros can be retained when the file is saved in csv format?

Any help would be appreciated.