"The United Nations and the International Energy Agency are clear: there is no safe level of new oil and gas extraction, any new projects commit us to global overheating and an un-liveable world," she wrote.
"Contrary to Shell's public expressions around Net Zero, and as most of you will know from the inside, Shell is not winding down oil and gas, but planning to explore and extract much more.
"It pains me to end this working relationship which I have greatly valued."
Dennett, a senior safety consultant at an agency that works with Shell, accused the oil and gas company of "double-talk" on climate, saying it operated "beyond the design limits of our planetary systems".
She also claimed that Shell was "not putting environmental safety before production".
She said: "Shell's stated safety ambition is to 'do no harm' – 'Goal Zero', they call it – and it sounds honourable but they are completely failing on it.
"They know that continued oil and gas extraction causes extreme harms, to our climate, to our environment and to people. And whatever they say, Shell is simply not winding down on fossil fuels."
Shell was a "major client" of Dennett's business Clout, which specialises in evaluating safety procedures in high-risk industries including oil and gas production. She began working with Shell in the aftermath of BP's Deepwater Horizon oil spill in 2010, which rocked the industry.
Shell's AGM takes place in London on Tuesday.
A Shell spokesman said: "Be in no doubt, we are determined to deliver on our global strategy to be a net zero company by 2050 and thousands of our people are working hard to achieve this.
"We have set targets for the short, medium and long term, and have every intention of hitting them.
"We're already investing billions of dollars in low-carbon energy, although the world will still need oil and gas for decades to come in sectors that can't be easily decarbonised."
