Theresa May has been forced to pledge she will maintain foreign aid spending if she wins the general election after pressure from Bill Gates and a former Archbishop of Canterbury.

The Conservative leader was effectively compelled to reveal part of the Tory election manifesto early amid a growing row over the issue.

But she faced criticism for ducking a similar commitment to the pensions ‘triple lock’, which currently guarantees that payments rise every year by either inflation, average earnings or 2.5 per cent, whichever is higher.

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In Washington the chancellor Philip Hammond also suggested that the Conservatives could ditch their 2015 commitment not to increase income tax, national insurance or VAT before 2020.

Mr Hammond was forced to scrap a plan raid on the self-employed earlier this year, after it emerged that his plans would break that pledge.

The Conservatives are not expected to unveil their election manifesto until next month.

But Mrs May is expected to dump a huge number of pledges made by her predecessor David Cameron and include her own priorities, including on grammar schools in England.

Earlier this week Mrs May was also forced to say she would re-commit her party to cut net migration to the tens of thousands.

Speaking in her constituency, where she also said that she would stand up to the SNP "separatists who want to break up our country”, Mrs May said that the pledge to spend 0.7 per cent of national income on foreign aid "remains and will remain".

But she hinted that ministers could change the definition of foreign aid.

Reports suggest the Conservatives are considering including military spending in the overall total.

On Monday Scottish Conservative leader Ruth Davidson said that the 0.7 per cent pledge showed that the UK would remain a “good, global citizen" as it left the European Union.

Mrs May also came under pressure over the issue from Microsoft co-founder and philanthropist Bill Gates, charities including Save the Children, Unicef and Oxfam, and the former Archbishop of Canterbury Rowan Williams.

Dr Williams described the pledge as a " badge of honour" and said its future would impact on whether the UK was seen as "Little Britain" or "Global Britain" after Brexit.

Charities welcomed the move Mrs May's pledge but it was criticised by the TaxPayers' Alliance pressure group.

The Liberal Democrats called on Mrs May to sign up to the OECD's definition of foreign aid spending to ensure the UK's contribution is not eroded “by the back door".

But she faced growing calls to spell out her plans for pensions, after failing to commit to preserving the "triple lock".

The Lib Dems accused the Tories of "getting their betrayal in early", while Labour leader Jeremy Corbyn highlighted his party's commitment.

Asked if older people could expect to see their pensions continue to rise if she wins the election, Mrs May said: "What I would say to pensioners is, just look what the Conservatives in government have done. Pensioners today are £1,250 better off as a result of action that has been taken.

"We were very clear about the need to support people in their old age, and that's exactly what we've done."

Earlier the Chancellor had hinted that the lock on tax rises could go as well.

He told the BBC: "All chancellors would prefer to have more flexibility in how they manage the economy and how they manage the overall tax burden down (rather) than having to have their hands constrained.

"But what we put in the manifesto will be decided in the next few days and we will publish that."

Shadow chancellor John McDonnell said: "Theresa May's refusal to commit the Tories to maintaining the pensions triple lock only further proves the Tories are abandoning older people. It's now clear pensions protections are now in jeopardy.

"Labour will stand up for older people by maintaining the pensions triple lock and by keeping the winter fuel allowance and free bus passes so that the elderly can go about their lives with the dignity they deserve."

Liberal Democrat frontbencher Alistair Carmichael said pensioners "should now realise that their pensions are not safe in Conservative hands".

"This is May and Hammond admitting that the cupboard is bare thanks to their disastrous hard Brexit," he added.

"Theresa May's refusal to guarantee the triple lock is yet another massive U-turn by the Prime Minister. Conservative high command is clearly in total disarray as it attempts a handbrake turn every day, including on such major issues as immigration, foreign aid and the single market."