diff --git a/src/APIClient.ts b/src/APIClient.ts
index 94058c10..692ad5ad 100644
--- a/src/APIClient.ts
+++ b/src/APIClient.ts
@@ -1,4 +1,4 @@
-import {RESTClient} from './client/RESTClient';
+import {Authorization, RESTClient} from './client/RESTClient';
 import {LightstreamerAPI} from './lightstreamer';
 
 export class APIClient {
@@ -8,7 +8,7 @@ export class APIClient {
   static URL_DEMO = 'https://demo-api.ig.com/gateway/deal/';
   static URL_LIVE = 'https://api.ig.com/gateway/deal/';
 
-  constructor(private readonly baseUrl: string, apiKey: string) {
+  constructor(private readonly baseUrl: string, apiKey: string | Authorization) {
     this.rest = new RESTClient(baseUrl, apiKey);
     this.stream = new LightstreamerAPI(this.rest.auth);
   }
diff --git a/src/client/RESTClient.ts b/src/client/RESTClient.ts
index 9eb2ee47..e138ddd5 100644
--- a/src/client/RESTClient.ts
+++ b/src/client/RESTClient.ts
@@ -8,8 +8,10 @@ import axiosRetry from 'axios-retry';
 export interface Authorization {
   accessToken?: string;
   accountId?: string;
+  apiKey?: string;
   clientSessionToken?: string;
   lightstreamerEndpoint?: string;
+  password?: string;
   refreshToken?: string;
   securityToken?: string;
   username?: string;
@@ -35,11 +37,19 @@ export class RESTClient {
   readonly httpClient: AxiosInstance;
   readonly auth: Authorization = {};
 
-  constructor(baseURL: string, private readonly apiKey: string) {
+  constructor(baseURL: string, private readonly apiKey: string | Authorization) {
     this.httpClient = axios.create({
       baseURL: baseURL,
     });
 
+    function isAuthorization(candidate: any): candidate is Authorization {
+      return typeof candidate !== 'string';
+    }
+
+    if (isAuthorization(this.apiKey)) {
+      this.auth = this.apiKey;
+    }
+
     function randomNum(min: number, max: number): number {
       return Math.floor(Math.random() * (max - min + 1) + min);
     }
@@ -71,7 +81,7 @@ export class RESTClient {
     this.httpClient.interceptors.request.use(async config => {
       const updatedHeaders = {
         ...config.headers,
-        'X-IG-API-KEY': this.apiKey,
+        'X-IG-API-KEY': isAuthorization(this.apiKey) ? this.apiKey.apiKey : this.apiKey,
       };
 
       const {accessToken, accountId, securityToken, clientSessionToken} = this.auth;
diff --git a/src/login/LoginAPI.test.ts b/src/login/LoginAPI.test.ts
index 81d31ae9..290cb981 100644
--- a/src/login/LoginAPI.test.ts
+++ b/src/login/LoginAPI.test.ts
@@ -90,6 +90,43 @@ describe('LoginAPI', () => {
       expect(switchAccountResponse.hasActiveLiveAccounts).toBe(true);
       expect(switchAccountResponse.trailingStopsEnabled).toBe(false);
     });
+
+    it('uses already existing credentials', async () => {
+      const username = 'top';
+      const password = 'secret';
+
+      nock(APIClient.URL_DEMO)
+        .post(LoginAPI.URL.SESSION)
+        .query(true)
+        .reply(200, (_: string, requestBody: Record<string, any>) => {
+          expect(requestBody.identifier).toBe(username);
+          expect(requestBody.password).toBe(password);
+          return JSON.stringify({
+            accountId: 'ABC123',
+            clientId: '133721337',
+            lightstreamerEndpoint: 'https://demo-apd.marketdatasystems.com',
+            oauthToken: {
+              access_token: '6ba8e2bd-1337-40e5-9299-68f60474f986',
+              expires_in: '60',
+              refresh_token: '83c056b8-1337-46d3-821d-92a1dffd7f1e',
+              scope: 'profile',
+              token_type: 'Bearer',
+            },
+            timezoneOffset: 1,
+          });
+        });
+
+      const apiClient = new APIClient(APIClient.URL_DEMO, {
+        apiKey: '123',
+        password,
+        username,
+      });
+      const getSessionToken = spyOn<LoginAPI>(apiClient.rest.login, 'getSessionToken').and.callFake(() =>
+        Promise.resolve(true)
+      );
+      await apiClient.rest.login.createSession();
+      expect(getSessionToken).toHaveBeenCalledTimes(1);
+    });
   });
 
   describe('setupSessionWithToken', () => {
diff --git a/src/login/LoginAPI.ts b/src/login/LoginAPI.ts
index 0dfb656c..0c9f0dcc 100644
--- a/src/login/LoginAPI.ts
+++ b/src/login/LoginAPI.ts
@@ -41,15 +41,15 @@ export class LoginAPI {
    * @param password - Password
    * @see https://labs.ig.com/rest-trading-api-reference/service-detail?id=534
    */
-  async createSession(username: string, password: string): Promise<TradingSession> {
+  async createSession(username?: string, password?: string): Promise<TradingSession> {
     delete this.auth.accessToken;
 
     const resource = LoginAPI.URL.SESSION;
     const response = await this.apiClient.post<TradingSession>(
       resource,
       {
-        identifier: username,
-        password,
+        identifier: this.auth.username || username,
+        password: this.auth.password || password,
       },
       {
         'axios-retry': {