Image caption The regular Army's numbers are being cut by 20,000 by 2020

Defence Secretary Philip Hammond has warned Tory MPs they risk "serious damage" to the UK's reserve forces if they vote against government plans to reorganise the Army.

The government wants to expand the Army Reserve to 30,000 to offset cuts of 20,000 regular troops by 2020.

But Tory MP John Baron has tabled an amendment to the coalition's bill, which would delay the plans until their impact is further discussed.

MPs debate the proposals on Wednesday.

Mr Hammond's remarks come in a letter - seen by the BBC - that he has written to Tory MPs in an attempt to avoid a defeat in the House of Commons.

In it, he urges his colleagues to vote against a rebel amendment to the Defence Reform Bill which would force the government to assess the "viability and cost-effectiveness" of the proposals and delay any reforms until the report had been supported by both MPs and peers.

'Wrong signal'

Conservatives behind the amendment fear there will not be enough reservists to fill the gaps left by cuts in the regulars and they are urging the government to publish up-to-date reservist recruitment numbers.

They fear false economies could damage the capability of the armed forces.

The amendment has been signed by more than 20 Tory MPs and and is backed by the DUP.

But in his letter Mr Hammond says this would be "damaging to the army and Britain's military capability".

He says: "I hope colleagues will support the government in resisting John's amendment. To do otherwise will not only give a significant fillip to the Labour Party but more significantly would risk serious damage to our future armed forces."

He says the amendment would halt the process of recruiting reservists, which "would send completely the wrong signal to those thinking of joining the reserves".

Mr Hammond claims the amendment would stop the name change from the Territorial Army to the Army Reserve, stop payments to small and medium-sized firms whose employees join the reserves, and stop reservists getting paid leave when training as well as on operations.

He adds: "Furthermore, there is a very significant risk that halting, or threatening to halt, the implementation of measures in the Reserves White Paper would hit morale among the existing reserve forces, confuse employers, and make future recruitment a much more difficult task."

Mr Hammond also accuses Labour of "a highly opportunistic move on their part" to support the rebels simply to inflict a defeat on the government, adding: "It is extremely regrettable that they are seeking to play politics with the future of our armed forces."

The letter shows that the government considers defeat a definite possibility. The result will be determined by how strongly Labour imposes its three-line whip and how many Tory MPs choose to abstain.

Image caption There are currently 19,000 trained reservists

Mr Baron said: "Rising costs and badly missed reserve recruitment targets confirm the Reserve plans are in trouble, and raise the real prospect of unacceptable capability gaps and false economies.

"We ought to revert to the original plan in 2011 to maintain the Regulars until the Reservists are ready to take their place.

"These are not wrecking amendments. If passed, they would neither derail the bill nor the government's Army Reserve plans but simply delay them until Parliament has scrutinised their viability and cost-effectiveness. The delay need not be long if the government allowed prompt scrutiny of their report.

"I am not calling upon the government to reverse course, but to recognise its plans as currently realised are flawed."

Labour's shadow defence secretary Vernon Coaker said: "Labour will work with the government when we think it is doing the right thing on defence.

"But we have a duty to voice our concerns, shared by MPs from all parties, that ministers are gambling with Britain's defence and security and the ability of our armed forces to protect our country.

"The government's defence reforms are a mess, and David Cameron and his defence secretary have nobody to blame but themselves."