Britain's Nationwide warns of market share squeeze as profit falls

* Underlying profit 588 mln stg (2016 H1: 615 mln)

* Competition set to squeeze mortgage market share in H2

* Low interest rates pressure returns (Adds CEO quote, details)

By Lawrence White and Emma Rumney

LONDON, Nov 17 (Reuters) - Britain's Nationwide Building Society reported a 4 percent fall in half-year profit on Friday, a second consecutive period of decline that it attributed to low interest rates and tough competition in the mortgage market.

The lender reported an underlying profit of 588 million pounds ($778 million) for the six months to September 30, down from 615 million a year earlier.

Britain's second-biggest provider of mortgages has been hit in recent years by falling home loans - its main product - and low interest rates, which have squeezed its margins.

Nationwide said those pressures were set to continue amid intensifying competition.

"We're prepared for the possibility that intense competition combined with declining consumer confidence may lead to a moderation in gross lending and market share in the second half of the year," Chief Executive Joe Garner said.

Rival lender Virgin Money on Thursday also warned its mortgage market share would be lower than previously expected, as low interest rates push banks to offer increasingly attractive offers to home buyers.

Nationwide has in recent months pared back its business model to focus on its core offering of mortgages and savings and current accounts, selling or closing other business lines like car insurance and inheritance tax planning.

The first-half results followed an 18 percent fall in profits in the first quarter and a 23 percent fall in annual profits last year.

Nationwide said its underlying profit however increased, once a 100 million pound one-off gain from asset sales in the corresponding period a year ago was taken into account.

($1 = 0.7557 pounds) (editing by Jason Neely and Adrian Croft)