LONDON (Reuters) - Britain's armed forces would be unable to defend the country against a serious military attack because they have been "withered" by cuts, a former senior commander warned before his retirement, the Financial Times reported on Saturday.

In a 10-page memo sent to Defense Secretary Michael Fallon in April, General Richard Barrons, who was head of Britain's Joint Forces Command, said there was no plan to defend the nation against a conventional attack and it would be overwhelmed by a concerted Russian air campaign.

"Capability that is foundational to all major armed forces has been withered by design," Barrons said in the private memo which the FT said it had obtained.

Last year, the government committed Britain to meeting NATO's defense spending pledge of two percent of GDP until the end of the decade, and in its five-year defense plan announced last November promised to increase spending by 12 billion pounds ($15.6 billion) to 178 billion over the next decade.

However, defense spending cuts in real terms since 2010 to help cut a record budget deficit has meant that the size of the armed forces has shrunk by around one sixth while last year's review envisaged a reduction in civilian staff of 30 percent.

(Reporting by Michael Holden)