The former couple - who have children Leni, seven, Henry, six, Johan, five, and two-year-old Lou - ended their seven-year marriage in January and started divorce proceedings earlier this month.
But Seal, 49, is said to be less-than-pleased about pictures which appear to show Heidi enjoying a romantic picnic date with 'Germany's Next Top Model' judge Thomas Hayo.
A source told Now magazine: ''Seal's been in a terrible mood for weeks and this has really set him off. He says they agreed not to start dating until the divorce was finalised.''
Heidi, 38, insists she and Thomas are just friends, although her estranged husband believes otherwise.
The insider added: Seal thinks there's no smoke without fire.''
Following the announcement of the couple's separation, the 'Kiss From A Rose' hitmaker revealed the pair had ''grown apart'', but insisted he still ''very much loved'' Heidi.
He explained: ''You just grow apart after a while. She still, in my opinion, the most wonderful woman in the world. I think our priority was to remain civil and do this thing with dignity. We still very much love each other.
''To say that neither of us were grieving would be an out and out lie, and I don't mind telling you that. I think the operative here is that we are civil to each other and we both have a tremendous amount of respect and love and we try and do this with as much dignity as we possibly can.''