PM had made the benefits pledge central to his plan to

David Cameron will fail to get his key EU demand to stop benefits for working migrants who have been in the country less than four years, British diplomats say.

One of the Government's most senior officials in the Brussels negotiations disclosed that the Prime Minister will secure welfare restrictions only for a matter of months.

The admission is a major setback for Mr Cameron who made the pledge the central part of his plan for using the renegotiation to reduce migration.

Prime Minister David Cameron with the European Commission president Jean-Claude Juncker. British diplomats say the PM will fail in his key EU demand to stop benefits for migrant workers who have been in the country less than four years

Behind closed doors, European Commission officials have signalled willingness to change EU rules so Britain is not penalised for being outside the eurozone. Leaders also backed the Prime Minister's call for the EU to be more competitive and do more to secure trade deals.

But Mr Cameron's demand to curb migrant benefits is being met with fierce resistance despite being the reform most keenly anticipated by the UK public.

A top British diplomat directly involved in the negotiations revealed it is 'more likely' the deal will include denying benefits 'for months rather than years'.

In a speech last November, Mr Cameron pledged to stop working migrants from receiving benefits for four years, as a red line for Britain to stay in the EU.

Last month, Foreign Secretary Philip Hammond, pictured, warned Mr Cameron's proposals on migrant benefits were the 'most sensitive' and 'most difficult' of the demands

He promised to 'negotiate a cut to EU migration and make welfare reform an absolute requirement', adding: 'We want to reduce the number of EU workers coming to the UK … that means reducing the incentives for lower-paid, low-skilled EU workers'.

The Prime Minister argued 'in-work benefits', such as tax credits that top up low wages, are a 'pull factor' for incomers to Britain.

He said: 'Someone coming to the UK from elsewhere in the EU, who is employed on the minimum wage and who has two children back in their home country, will receive around £700 per month in benefits … more than twice what they would receive in Germany, and three times more than in France.'

Mr Cameron added: 'I will insist that in the future those who want to claim tax credits and child benefit must live here and contribute to our country for a minimum of four years.' He indicated his readiness to campaign for an 'Out' vote if he did not achieve the changes, saying: 'If I succeed, I will … campaign to keep this country in a reformed EU. If our concerns fall on deaf ears … then of course I rule nothing out.'

In the speech at a JCB factory in Staffordshire, Mr Cameron joked his migration reforms had 'set off alarm bells' in Brussels after he was interrupted by a siren.

But Eastern European leaders voiced opposition to the plan, which they claim discriminates against workers from their countries.

Last month, Foreign Secretary Philip Hammond warned Mr Cameron's proposals on migrant benefits were the 'most sensitive' and 'most difficult' of the demands. He added that the migrant crisis had made talks more 'complicated'.

On Tuesday, Mr Hammond again tried to dampen expectations during Foreign Office questions, saying treaty change, automatic primacy of Parliament and deals over freedom of movement were unlikely. Mr Cameron is under pressure to deliver the welfare change, to get migration numbers under control.

Read more of the latest news on David Cameron and his demand to stop migrant benefits

Ukip leader Nigel Farage says that unless David Cameron can get a full-on treaty change, his renegotiation will be deemed a complete failure

Ukip leader Nigel Farage said: 'Unless David Cameron can get full-on treaty change, stop open borders and ban EU migrants dipping into the British welfare pot then his negotiation will be deemed a total failure.'

British officials have been holding 'technical talks' with Brussels counterparts since June to begin work on the renegotiation. But Mr Cameron has tried to avoid details of any progress being made public.

Last week, he agreed to put his demands in writing at the start of next month, ahead of a summit of all 28 EU members in December.