Upgrade from 13.04 to 13.10 fails due to grep:i386

[Andrew Love]

Software Updater prompted me that release 13.10 was available.
Selected Upgrade
Upgrade wizard notified me it could not continue and gave some possible causes, mainly based around the idea that I may have a pre-release installed.
I do not have any pre-release versions of Ubuntu installed, just a regular 64bit 13.04.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: ubuntu-release-upgrader-core 1:0.192.13
ProcVersionSignature: Ubuntu 3.8.0-32.47-generic 3.8.13.10
Uname: Linux 3.8.0-32-generic x86_64
ApportVersion: 2.9.2-0ubuntu8.4
Architecture: amd64
CrashDB: ubuntu
Date: Thu Oct 17 17:33:16 2013
InstallationDate: Installed on 2012-04-27 (538 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to raring on 2013-10-17 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

2013-10-17 17:32:43,807 DEBUG Marking 'ubuntu-desktop' for upgrade
2013-10-17 17:32:43,918 DEBUG The package 'grep:i386' is marked for removal but it's an ESSENTIAL package

That's weird.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Brian Murray (brian-murray)]

This is interesting:

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb [arch=amd64,i386] http://gb.archive.ubuntu.com/ubuntu/ raring main restricted
deb-src [arch=amd64,i386] http://gb.archive.ubuntu.com/ubuntu/ raring main restricted

[Fred Schnittke (fschnittke)]

Fails upgrading to 13.10:
/var/log/dist-upgrade/main.log
DEBUG The package 'grep:i386' is marked for removal but it's an ESSENTIAL package
ERROR Dist-upgrade failed: 'The essential package 'grep:i386' is marked for removal.'