The Office for National Statistics said there were only 30 instances of British firms buying other British companies worth more than 1 million pounds, down from 46 in the second quarter.

The total value of the acquisitions rose to 2.9 billion pounds -- the highest since the first quarter of 2013 -- from 1.3 billion pounds in the second quarter.

The bulk of this comprised a merger between electronics stores Carphone Warehouse and Dixons Retail, which concluded in August with the formation of Dixons Carphone.

Foreign companies bought 19 British companies in the third quarter, matching a 25-year low set in the first three months of 2013.

The ONS did not provide reasons for the weak merger activity, but cited other surveys and research papers.

An OECD paper last month suggested weak foreign direct investment between European Union countries was due to a lack of economic confidence, though a Thomson Reuters study suggested that global M&A activity this year was its highest since 2007.

(Reporting by Andy Bruce; Editing by Catherine Evans)