diff --git a/gui/ice40/mainwindow.cc b/gui/ice40/mainwindow.cc
index 934798bb..e36464a0 100644
--- a/gui/ice40/mainwindow.cc
+++ b/gui/ice40/mainwindow.cc
@@ -17,18 +17,24 @@ MainWindow::MainWindow(Context *_ctx, QWidget *parent)
     std::string title = "nextpnr-ice40 - " + ctx->getChipName();
     setWindowTitle(title.c_str());
 
-    createMenu();
-
     task = new TaskManager(_ctx);
     connect(task, SIGNAL(log(std::string)), this, SLOT(writeInfo(std::string)));
+
+    createMenu();
 }
 
-MainWindow::~MainWindow() {}
+MainWindow::~MainWindow() { delete task; }
 
 void MainWindow::createMenu()
 {
     QMenu *menu_Custom = new QMenu("&ICE 40", menuBar);
     menuBar->addAction(menu_Custom->menuAction());
+
+    QAction *actionTerminate = new QAction("Terminate", this);
+    actionTerminate->setStatusTip("Terminate running task");
+    connect(actionTerminate, SIGNAL(triggered()), task,
+            SLOT(terminate_thread()));
+    menu_Custom->addAction(actionTerminate);
 }
 
 void MainWindow::open()
@@ -39,7 +45,7 @@ void MainWindow::open()
         tabWidget->setCurrentWidget(info);
 
         std::string fn = fileName.toStdString();
-        task->parsejson(fn);
+        Q_EMIT task->parsejson(fn);
     }
 }
 bool MainWindow::save() { return false; }
\ No newline at end of file
diff --git a/gui/ice40/worker.cc b/gui/ice40/worker.cc
index 5702137b..a309f868 100644
--- a/gui/ice40/worker.cc
+++ b/gui/ice40/worker.cc
@@ -10,9 +10,19 @@
 #include "route.h"
 #include "timing.h"
 
-Worker::Worker(Context *_ctx) : ctx(_ctx)
+struct WorkerInterruptionRequested
 {
-    log_write_function = [this](std::string text) { Q_EMIT log(text); };
+};
+
+Worker::Worker(Context *_ctx, TaskManager *parent) : ctx(_ctx)
+{
+    log_write_function = [this, parent](std::string text) {
+        Q_EMIT log(text);
+        if (parent->shouldTerminate()) {
+            parent->clearTerminate();
+            throw WorkerInterruptionRequested();
+        }
+    };
 }
 
 void Worker::parsejson(const std::string &filename)
@@ -32,14 +42,16 @@ void Worker::parsejson(const std::string &filename)
             log_error("Placing design failed.\n");
         if (!route_design(ctx))
             log_error("Routing design failed.\n");
-        Q_EMIT log("done");
+        Q_EMIT log("DONE\n");
     } catch (log_execution_error_exception) {
+    } catch (WorkerInterruptionRequested) {
+        Q_EMIT log("CANCELED\n");
     }
 }
 
-TaskManager::TaskManager(Context *ctx)
+TaskManager::TaskManager(Context *ctx) : toTerminate(false)
 {
-    Worker *worker = new Worker(ctx);
+    Worker *worker = new Worker(ctx, this);
     worker->moveToThread(&workerThread);
     connect(&workerThread, &QThread::finished, worker, &QObject::deleteLater);
     connect(this, &TaskManager::parsejson, worker, &Worker::parsejson);
@@ -53,4 +65,22 @@ TaskManager::~TaskManager()
     workerThread.wait();
 }
 
-void TaskManager::info(const std::string &result) { Q_EMIT log(result); }
\ No newline at end of file
+void TaskManager::info(const std::string &result) { Q_EMIT log(result); }
+
+void TaskManager::terminate_thread()
+{
+    QMutexLocker locker(&mutex);
+    toTerminate = true;
+}
+
+bool TaskManager::shouldTerminate()
+{
+    QMutexLocker locker(&mutex);
+    return toTerminate;
+}
+
+void TaskManager::clearTerminate()
+{
+    QMutexLocker locker(&mutex);
+    toTerminate = false;
+}
\ No newline at end of file
diff --git a/gui/ice40/worker.h b/gui/ice40/worker.h
index 12d740dd..a2d220bd 100644
--- a/gui/ice40/worker.h
+++ b/gui/ice40/worker.h
@@ -1,17 +1,20 @@
 #ifndef WORKER_H
 #define WORKER_H
 
+#include <QMutex>
 #include <QThread>
 #include "nextpnr.h"
 
 // FIXME
 USING_NEXTPNR_NAMESPACE
 
+class TaskManager;
+
 class Worker : public QObject
 {
     Q_OBJECT
   public:
-    Worker(Context *ctx);
+    Worker(Context *ctx, TaskManager *parent);
   public Q_SLOTS:
     void parsejson(const std::string &filename);
   Q_SIGNALS:
@@ -29,11 +32,19 @@ class TaskManager : public QObject
   public:
     TaskManager(Context *ctx);
     ~TaskManager();
+    bool shouldTerminate();
+    void clearTerminate();
   public Q_SLOTS:
     void info(const std::string &text);
+    void terminate_thread();
   Q_SIGNALS:
+    void terminate();
     void parsejson(const std::string &);
     void log(const std::string &text);
+
+  private:
+    QMutex mutex;
+    bool toTerminate;
 };
 
 #endif // WORKER_H