Heather, played by Cheryl Fergison, will be killed by Ben Mitchell during a heated argument next month, but police will suspect the wrong man.
Ben confesses his crime to dad Phil, who keeps an eye on the police investigation and persuades his son to keep quiet.
"Heather's murder is the next really big storyline coming up - and it's going to be huge. Everyone was desperate to keep the fact that it's Ben who kills her under wraps because it's all pointing towards Andrew," a source told the Daily Star Sunday.
"He's been getting more controlling and possessive over the last few weeks and has a bad temper.
"So even though it's Ben who kills her, the police and everyone else on Albert Square believe it was Andrew. It's a terrible crime that sends shockwaves through the Square."
It was announced last year that Fergison would be leaving EastEnders after five years.
She filmed her final scenes last month and her co-stars will film Heather's funeral this week.
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Watch Heather's EastEnders character trail below: