diff --git a/libreplan-business/src/main/java/org/libreplan/business/resources/entities/Worker.java b/libreplan-business/src/main/java/org/libreplan/business/resources/entities/Worker.java
index 23ea4aee8..0a9a4b9af 100644
--- a/libreplan-business/src/main/java/org/libreplan/business/resources/entities/Worker.java
+++ b/libreplan-business/src/main/java/org/libreplan/business/resources/entities/Worker.java
@@ -27,6 +27,7 @@ import org.hibernate.validator.AssertTrue;
 import org.hibernate.validator.NotEmpty;
 import org.libreplan.business.common.Registry;
 import org.libreplan.business.common.exceptions.InstanceNotFoundException;
+import org.libreplan.business.users.daos.IUserDAO;
 import org.libreplan.business.users.entities.User;
 
 /**
@@ -214,4 +215,28 @@ public class Worker extends Resource {
         this.user = user;
     }
 
+    @AssertTrue(message = "User already bound to other worker")
+    public boolean checkUserNotBoundToOtherWorker() {
+        if (user == null || user.isNewObject()) {
+            return true;
+        }
+
+        IUserDAO userDAO = Registry.getUserDAO();
+        User foundUser = userDAO.findOnAnotherTransaction(user.getId());
+        if (foundUser == null) {
+            return true;
+        }
+
+        Worker worker = foundUser.getWorker();
+        if (worker == null) {
+            return true;
+        }
+
+        if (getId() == null) {
+            return false;
+        }
+
+        return getId().equals(worker.getId());
+    }
+
 }
diff --git a/libreplan-business/src/main/java/org/libreplan/business/users/daos/IUserDAO.java b/libreplan-business/src/main/java/org/libreplan/business/users/daos/IUserDAO.java
index 5a555f502..d9b6aefa2 100644
--- a/libreplan-business/src/main/java/org/libreplan/business/users/daos/IUserDAO.java
+++ b/libreplan-business/src/main/java/org/libreplan/business/users/daos/IUserDAO.java
@@ -92,4 +92,6 @@ public interface IUserDAO extends IGenericDAO<User, Long>{
      */
     List<User> getUnboundUsers(Worker worker);
 
+    User findOnAnotherTransaction(Long id);
+
 }
diff --git a/libreplan-business/src/main/java/org/libreplan/business/users/daos/UserDAO.java b/libreplan-business/src/main/java/org/libreplan/business/users/daos/UserDAO.java
index ed11bff4f..680b38d12 100644
--- a/libreplan-business/src/main/java/org/libreplan/business/users/daos/UserDAO.java
+++ b/libreplan-business/src/main/java/org/libreplan/business/users/daos/UserDAO.java
@@ -149,4 +149,14 @@ public class UserDAO extends GenericDAOHibernate<User, Long>
                 org.hibernate.criterion.Order.asc("loginName")).list();
     }
 
+    @Override
+    @Transactional(readOnly = true, propagation = Propagation.REQUIRES_NEW)
+    public User findOnAnotherTransaction(Long id) {
+        try {
+            return find(id);
+        } catch (InstanceNotFoundException e) {
+            return null;
+        }
+    }
+
 }
diff --git a/libreplan-business/src/test/java/org/libreplan/business/test/resources/daos/ResourceDAOTest.java b/libreplan-business/src/test/java/org/libreplan/business/test/resources/daos/ResourceDAOTest.java
index e52d4aed1..35ad7b0fe 100644
--- a/libreplan-business/src/test/java/org/libreplan/business/test/resources/daos/ResourceDAOTest.java
+++ b/libreplan-business/src/test/java/org/libreplan/business/test/resources/daos/ResourceDAOTest.java
@@ -3,7 +3,7 @@
  *
  * Copyright (C) 2009-2010 Fundación para o Fomento da Calidade Industrial e
  *                         Desenvolvemento Tecnolóxico de Galicia
- * Copyright (C) 2010-2011 Igalia, S.L.
+ * Copyright (C) 2010-2012 Igalia, S.L.
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -46,6 +46,7 @@ import org.libreplan.business.calendars.entities.ResourceCalendar;
 import org.libreplan.business.common.IAdHocTransactionService;
 import org.libreplan.business.common.IOnTransaction;
 import org.libreplan.business.common.exceptions.InstanceNotFoundException;
+import org.libreplan.business.common.exceptions.ValidationException;
 import org.libreplan.business.resources.daos.ICriterionDAO;
 import org.libreplan.business.resources.daos.ICriterionTypeDAO;
 import org.libreplan.business.resources.daos.IResourceDAO;
@@ -56,6 +57,8 @@ import org.libreplan.business.resources.entities.CriterionType;
 import org.libreplan.business.resources.entities.Interval;
 import org.libreplan.business.resources.entities.Resource;
 import org.libreplan.business.resources.entities.Worker;
+import org.libreplan.business.users.daos.IUserDAO;
+import org.libreplan.business.users.entities.User;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.test.annotation.NotTransactional;
 import org.springframework.test.context.ContextConfiguration;
@@ -91,6 +94,9 @@ public class ResourceDAOTest {
     @Autowired
     private ICriterionTypeDAO criterionTypeDAO;
 
+    @Autowired
+    private IUserDAO userDAO;
+
     @Test
     public void saveResourceWithCalendar() throws InstanceNotFoundException {
         Resource resource = givenValidWorker();
@@ -233,4 +239,44 @@ public class ResourceDAOTest {
         assertThat(result.size(), not(equalTo(1)));
     }
 
+    private User givenStoredUser() {
+        return transactionService
+                .runOnAnotherTransaction(new IOnTransaction<User>() {
+
+                    @Override
+                    public User execute() {
+                        User user = User.create("login" + UUID.randomUUID(),
+                                "password", null);
+                        userDAO.save(user);
+                        user.dontPoseAsTransientObjectAnymore();
+                        return user;
+                    }
+                });
+    }
+
+    @Test(expected = ValidationException.class)
+    public void testWorkerBoundToUserAlreadyBound() {
+        final User user = givenStoredUser();
+
+        transactionService.runOnAnotherTransaction(new IOnTransaction<Void>() {
+            @Override
+            public Void execute() {
+                Worker worker1 = givenValidWorker();
+                worker1.setUser(user);
+                resourceDAO.save(worker1);
+                return null;
+            }
+        });
+
+        transactionService.runOnAnotherTransaction(new IOnTransaction<Void>() {
+            @Override
+            public Void execute() {
+                Worker worker2 = givenValidWorker();
+                worker2.setUser(user);
+                resourceDAO.save(worker2);
+                return null;
+            }
+        });
+    }
+
 }