Home Secretary Theresa May is planning tough new rules to curb the number of foreigners able to come to the UK on family visas, it was revealed on Sunday.

In a leaked letter obtained by The Sunday Telegraph, Mrs May proposed almost doubling the minimum income requirement to £25,700 for anyone wanting to bring a spouse, partner or dependent into Britain from outside the European Union.

She is also suggesting a longer probationary period, from two to five years, before spouses and partners can apply to live permanently in Britain.

The moves, to take effect from June, were set out in a letter to Deputy Prime Minister Nick Clegg in his role as chairman of the Cabinet home affairs sub-committee.

They are likely to face resistance from the Liberal Democrats, who traditionally take a softer line on immigration than their Conservative coalition partners.

However, the coalition Government is off course for meeting Prime Minister David Cameron's pledge to reduce net immigration to the tens of thousands by the end of the parliament.

It is thought the changes proposed by the Home Secretary could reduce the number of immigrants to Britain by 15,000 a year.

Mrs May wrote that the package she is proposing would "reduce the burdens on the taxpayer, promote integration and tackle abuse".

"In particular I propose a minimum income threshold of £25,700 for a British citizen or person settled in the UK to sponsor the settlement of a spouse or partner of non-EEA (European Economic Area) nationality," she wrote.

That would rise to up to £62,600 if there were children involved as well.

In an indication of the Government's desire to crack down on sham relationships among would-be immigrants, the Home Secretary referred also to the need to "differentiate between genuine and non-genuine relationships".

The current minimum income threshold is equivalent to £13,700 before tax. The independent Migration Advisory Committee last year suggested raising the figure to between £18,600 and £25,700.

A Home Office spokesman said: "We do not comment on leaked documents."