Narrative:

I was the first officer and pilot monitoring for this flight. Near the end of our taxi it was a rather busy time due to our starting the second engine followed by the before takeoff check. As I went off comm to make the departure PA I had the VHF 1 comm button pressed down so I could still hear ATC. Just before I keyed the mic to make my PA call I heard tower call us and thought I heard them say; 'XXXX cleared for take off runway xx; landing traffic 13 mile final.' so I went ahead and made my PA telling the passengers '...we are cleared for take off; flight attendants please prepare the cabin for departure.' when I returned to VHF 1 I completed the before takeoff checklist; verified we were entering the correct runway and also verified with the captain that we were cleared for take off which I thought I had heard tower say and the captain replied in the affirmative. We began our take off roll and somewhere around 100kts tower called; 'XXXX you do not have a take off clearance!' I responded; 'you want us to abort?' and tower replied; 'affirmative!' the aircraft reached a max IAS of 115-120kts during the rejected take off. The captain/pilot flying brought the aircraft to a stop and taxied clear of the runway uneventfully. I ran the table for the vmbe in the QRH and found our vmbe = 183 which was well below our max IAS. Our brake temperature monitoring system (btms) indications did not exceed a 4 on any one brake. We then contacted maintenance and had them inspect our brakes and fuse plugs before departure. Both the captain and I; the first officer understood we were cleared for takeoff. We both heard it on the radio and even verified it with each other. It was a busy time between checklists and receiving our clearance from ATC. Since I was still monitoring VHF 1 while making my PA I did not think to question what I thought I had heard.even if monitoring VHF 1 while off frequency the pilot that was off frequency should still confirm with ATC a take off clearance and not just verify it with the other pilot.

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Original NASA ASRS Text

Title: A CRJ-700 Captain delayed the engine start during taxi; and while starting it near the Runway's end; the First Officer seated the flight attendants. ATC then cleared the aircraft to line up and wait but as the Captain taxied onto the runway the First Officer; who did not hear line up; began the Takeoff Checklist. The Captain began the takeoff was told to reject around 100kts.

Narrative: I was the First Officer and Pilot Monitoring for this flight. Near the end of our taxi it was a rather busy time due to our starting the second engine followed by the Before Takeoff Check. As I went off comm to make the departure PA I had the VHF 1 comm button pressed down so I could still hear ATC. Just before I keyed the mic to make my PA call I heard Tower call us and thought I heard them say; 'XXXX cleared for Take Off runway XX; landing traffic 13 mile final.' So I went ahead and made my PA telling the passengers '...we are cleared for Take Off; flight attendants please prepare the cabin for departure.' When I returned to VHF 1 I completed the before takeoff checklist; verified we were entering the correct runway and also verified with the Captain that we were cleared for Take Off which I thought I had heard Tower say and the Captain replied in the affirmative. We began our Take Off roll and somewhere around 100kts Tower called; 'XXXX you do not have a Take Off clearance!' I responded; 'You want us to abort?' and Tower replied; 'Affirmative!' The aircraft reached a max IAS of 115-120kts during the rejected Take Off. The Captain/Pilot Flying brought the aircraft to a stop and taxied clear of the runway uneventfully. I ran the table for the Vmbe in the QRH and found our Vmbe = 183 which was well below our Max IAS. Our Brake Temperature Monitoring System (BTMS) indications did not exceed a 4 on any one brake. We then contacted Maintenance and had them inspect our brakes and fuse plugs before departure. Both the Captain and I; the First Officer understood we were cleared for Takeoff. We both heard it on the radio and even verified it with each other. It was a busy time between checklists and receiving our clearance from ATC. Since I was still monitoring VHF 1 while making my PA I did not think to question what I thought I had heard.Even if monitoring VHF 1 while off frequency the pilot that was off frequency should still confirm with ATC a Take Off clearance and not just verify it with the other pilot.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.