LONDON (Reuters) - Domino's Pizza, Britain's biggest pizza delivery firm, posted a 4 percent rise in third quarter underlying UK sales and said its revised expansion plans for Germany were on track.

Domino's said on Wednesday like-for-like sales at its 669 UK stores had risen 4 percent over the 13 weeks to September 29, driven by rising online and mobile demand.

That represented a slowdown from the 6.4 percent achieved in the first half of the year, when new products and promotions boosted sales, but was above analyst forecasts which had predicted a rise of 2.5-3 percent.

Total sales for the period rose 10.4 percent to 140.9 million pounds ($228.3 million). The company said it intends to open 50 stores in the current financial year.

The group, which makes almost all of its sales of pizzas such as Pepperoni Passion and Meatilicious in the UK and Ireland, has big plans for expansion into Germany but in July was forced to slow them, as higher labour costs meant it would break even there as much as two years later than hoped.

To help improve its performance there Domino's is handing the running of its poor performing own-managed stores over to more experienced franchisees, a model that is behind its success in the UK.

Domino's said on Wednesday it was confident that a good number of those transfers would be complete by the year end.

Like-for-like sales over the period in Germany climbed 9.4 percent, while sales in Ireland marked nine periods of consecutive growth with a rise of 2.4 percent.

Domino's chief executive Lance Batchelor said the group remained on track to meet full-year forecasts. The company is on average expected to report a pretax profit of 48.67 million pounds, according to a Reuters poll of eight analysts.

Shares in Domino's closed at 589 pence on Tuesday, up 10.2 percent on a year ago, valuing the business at around 968 million pounds.

(Reporting by Brenda Goh; Editing by Neil Maidment)