HP ZBook 15 starts automatically upon opening the lid

Ubuntu 14.04.3 on my HP ZBook 15 is the only system installed on the notebook. The BIOS version is the latest one available:
L70 Ver. 01.33
In BIOS I use the options:
* UEFI BIOS set to native UEFI
* Secure Boot On
* Hyperthreading Off

How to reproduce the bug:

1.) Shut down the operating system.
2.) Close the lid once the laptop is off.
3.) Open the lid.

The laptop starts and boots automatically, i.e. without pressing the power button. For me this is an inconvenient behaviour because the system may start when I accidentally partially open the lid while transporting the notebook.

I add that first I thought it might have had something to do with my (previous) temporary setting
intel_pstate=disable
in /etc/default/grub
However, after re-enabling intel_pstate (and running sudo update-grub and rebooting the system) the behaviour is unchanged: the notebook starts without pressing the power button.

The sleep regime works as it should: closing the lid forces the sleep state. Opening the lid does not wake up the OS. Power button has to be pressed to wake up the system.

I am sorry if I am not reporting the right package.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: grub-efi-amd64-signed 1.34.5+2.02~beta2-9ubuntu1.4
ProcVersionSignature: Ubuntu 3.19.0-33.38~14.04.1-generic 3.19.8-ckt7
Uname: Linux 3.19.0-33-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.18
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Nov 18 22:06:55 2015
InstallationDate: Installed on 2015-10-26 (23 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
SourcePackage: grub2-signed
UpgradeStatus: No upgrade log present (probably fresh install)